Investigation of Synthetic and Natural Lubricants

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(Under the direction of Prof. Wendy E. Krause and Prof. Alan E. Tonelli.)

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by Jing Liang

A dissertation submitted to the Graduate Faculty of North Carolina State University

in partial fullfillment of the requirements for the Degree of

Doctor of Philosophy

Fiber and Polymer Science

Raleigh, North Carolina

2008

APPROVED BY:

Prof. Sam M. Hudson Prof. Jan Genzer

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DEDICATION

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BIOGRAPHY

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ACKNOWLEDGEMENTS

First of all, I would like to thank my advisor Prof. Wendy E. Krause for her long-time encouragement and support. She has been a great mentor and teacher to me. It was a pleasure for me to be a part of our stimulating discussions.

I am also thankful to all current members of our research group, namely Rebecca Klossner, Hongyi Liu, and Junlong Song.

Thanks to my committee members, Dr. Alan E. Tonelli, Dr. Jan Genzer, and Dr. Sam M. Hudson. I also like to thank Dr. Orlando Rojas for providing me a lot of information and help.

I would like to thank the College of Textiles at large: faculty, staff and fellow students, especially to Angie Brantley, Carolyn Krystoff, Judy D. Elson, and Shane J. Jarvis

Thanks for the financial support from the National Textile Center and College of Textiles.

Furthermore, I want to thank my friends Huiqing Liu, Yao Liu who helped me a lot during my pregnancy and have been my best friends since I started my study at NCSU.

Special thanks goes to my husband Cheng Wang for his love and unlimited sup-port. I also want to thank my lovely son, Daniel Wang who brings me so much fun and makes me happy.

Appreciation to my parents in law Xuelian Niu and Zhizhong Wang for their great support and help during my thesis work.

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LIST OF FIGURES

Figure 2.1 Picture of the nanoindenter instrument. . . 4

Figure 2.2 Schematic of 2D (lateral force) Transducer. . . 5

Figure 2.3 Raw data of coefficient of friction vs. displacement of polypropylene a)under different loads; b)measured at three different locations on the same surface under one constant normal load(5, 40 and 100 µN, as labeled on plot) by fCon-100. . . 9

Figure 2.4 AFM image of scratching tests on polyethylene (MW: 1,800) a) scratch-ing test by Berk-50nm. The loadscratch-ing from left to right, are 40, 50, 60, 70, 80 and 90 µN; b) scratching test by Con-50, the loading from left to right, are 90, 100, 110, 130,150, 170 and 190 µN.. . . 12 Figure 2.5 Coefficient of friction vs. load of polyethylene (PE9, MW: 900, PE18,

MW: 1800) using Berk-50nm and Con-50 tip. The dashed lines were the repeated experiment results of the solid lines. Different coefficient of friction measured by two different tips illustrates different friction mechanism involved in scratching process. . . 13

Figure 2.6 Coefficient of friction vs. load of PE (PE 9, MW: 900), PE (PE18, MW: 1,800) and PP surface using Con-50. The dashed lines were the repeated experimental results of the solid lines. Different coefficient of friction between PE and PP shows that different microstructure may give rise to different sliding resistance. . . 15

Figure 2.7 a): Coefficient of friction vs. load of PE (PE65, MW: 6,500), PP and cellulose surface measured by using fCon-100 b): Coefficient of friction vs. load of polypropylene surface with and without water measured by using fCon-100 tip; c): Coefficient of friction vs. load of polyethylene (MW: 6,500) surface with and without measured by water using fCon-100 tip; d): Coefficient of friction vs. load of cellulose surface with and without water measured by using fCon-100 tip. . . 17

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Figure 3.2 The molecular structure of UCON series . . . 24

Figure 3.3 The molecular structure of Pluronic series . . . 25

Figure 3.4 a) Coefficient of friction vs. load of polypropylene surface using UCON series [UCON-12 (PPO13PEO12), UCON-16 (PPO13PEO17), UCON-39 (PPO33 -PEO44)] as lubricants; b) Coefficient of friction vs. load of cellulose surface us-ing Pluronic series [Pluronic-84 (PEO76PPO29PEO76), Pluronic-34 (PEO19PPO29 -PEO19), Pluronic-65 (PEO37PPO56PEO37)] as lubricants. . . 28 Figure 3.5 a) Coefficient of friction vs. load of polyethylene surface using UCON

series [UCON-12 (PPO13PEO12), UCON-16 (PPO13PEO17), UCON-39 (PPO33 -PEO44)] as lubricants; b) Coefficient of friction vs. load of cellulose surface using Pluronic series [Pluronic-84 (PEO76PPO29PEO76), Pluronic-34 (PEO19 -PPO29PEO19), Pluronic-65 (PEO37PPO56PEO37)] as lubricants. . . 29 Figure 3.6 a) Coefficient of friction vs. load of cellulose surface using UCON series

[UCON-12 (PPO13PEO12), UCON-16 (PPO13PEO17), UCON-39 (PPO33PEO44)] as lubricants; b) Coefficient of friction vs. load of cellulose surface using Pluronic series [Pluronic-84 (PEO76PPO29PEO76), Pluronic-34 (PEO19PPO29 -PEO19), Pluronic-65 (PEO37PPO56PEO37)] as lubricants. . . 31 Figure 3.7 Illustration of lubricant molecule on hydrophobic polymer surface. The

hydrophobic parts (PPO) adhere to polymer surface and hydrophilic parts (PEO) extend into the solution, and the more PPO groups, the stronger of the adsorption. . . 33

Figure 3.8 Adsorption isotherms for nonionic polymers adsorbed on polypropylene surfaces. The polymer solution flow rate was kept constant at 0.1 ml/min. . . 36

Figure 4.1 The structure of a synovial joint. . . 41

Figure 4.2 Rheopexy of the synovial fluid model. . . 43

Figure 4.3 Schematic diagram of adult articular cartilage showing zonal structure. 45

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Figure 4.5 Structure of hyaluronic acid. . . 62

Figure 4.6 Influence of shear rate on the specific viscosity at different polymer concentrations. . . 65

Figure 4.7 Proposed model of reversible phospholipid (PL) binding to HA in which the polar cationic head of PL (represented by a solid circle) interacts with the anionic glucuronyl carboxyl group, and the hydrocarbon fatty acid chains (depicted by wavy lines) orient themselves in such a manner that they can interact with the hydrophobic pockets present along the HA molecule. On binding to PL, the intra-chain coupling within HA polymer A (as shown by parallel bars) is disrupted to give B. This process allows the closed loops to open resulting in increased flexibility of the HA chains. While only part of one HA chian is shown, the coupled loops could enclose loops of other HA chains resulting in an extensive three-dimensional network in solution. . . 68

Figure 5.1 Typical time-dependent response of the macroscopic friction coefficient, showing µmin and µeq. . . 79 Figure 5.2 Coefficient of friction vs. load of polyethylene using hyaluronate (MW:

2.0 MDa) at different concentrations (0.7, 2, 3 and 5 mg/mL) as lubricants. . 85

Figure 5.3 Coefficient of friction vs. load of polyethylene using different molecu-lar weight hyaluronate (MW: 0.42, 0.78, 1.07 and 2.0 MDa) at 3 mg/mL as lubricants. . . 86

Figure 5.4 Coefficient of friction vs. load of polyethylene using albumin (bovine serum albumin, BSA 11 mg/mL) mixed with γ-globulins (γ-G) of different concentrations (7, 15, 20 and 28 mg/mL) as lubricants. . . 88

Figure 5.5 Coefficient of friction vs. load of polyethylene using albumin (bovine serum albumin, BSA 11 mg/mL), andγ-globulins (γ-G) of different concentra-tions (0, 7, 15, 20 and 28 mg/mL) mixed with HA (MW: 2.0 MDa, 3 mg/mL) as lubricants. . . 89

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Figure 5.7 Coefficient of friction vs. load of polyethylene using γ-globulins (γ-G, 7 mg/mL), and albumin (bovine serum albumin, BSA) of different concen-trations (0, 11, 20 mg/mL) mixed with HA (MW: 2.0 MDa, 3 mg/mL) as lubricants. . . 91

Figure 5.8 Coefficient of friction vs. load of polyethylene using DPPC (dipalmitoyl phosphatidylcholine) of different concentrations (in PG) (0.2 mg/mL, 3.55 mg/mL). . . 94

Figure 5.9 Coefficient of friction vs. load of polyethylene using DPPC (dipalmitoyl phosphatidylcholine) at different concentrations (0.2 mg/mL, 3.55 mg/mL) mixed with HA (2.0 MDa, 3 mg/mL) as lubricants. . . 95

Figure 5.10Coefficient of friction vs. load of polyethylene using a) DPPC (di-palmitoyl phosphatidylcholine) 0.2 mg/mL mixed with and without HA (2.0 Da, 3 mg/mL) as lubricants; b) DPPC (dipalmitoyl phosphatidylcholine) 3.55 mg/mL mixed with and without HA (2.0 MDa, 3 mg/mL) as lubricants. . . 96

Figure 6.1 The structure of D-Penicillamine. . . 100

Figure 6.2 Time effect on viscosity of NaHA with D-Penicillamine (at different concentrations). D-Penicillamine has a complex, time dependent effect on the viscosity of NaHA solutions-reducing the zero shear rate viscosity of a 3 mg/mL NaHA in PBS by ca. 40% after 44 days. . . 101 Figure 6.3 Oxidized D-Penicillamine, the dimer. . . 102

Figure 6.4 General frictional behavior of liquid-lubricated textile yarns. . . 103

Figure 6.5 Specific viscosity versus concentrations a) UCON series b) Pluronic series. . . 105

Figure 6.6 Viscosity versus shear rate of UCON-12 lubricant at different concen-trations (c = 100%, 90%, and 60%) and using stainless steel (SS) and poly-carbonate (PC) plates. Filled symbols are results using polypoly-carbonate plate. Open ones are results using steel plate. No change is observed between SS and PC Plates . . . 106

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Figure 6.8 Viscosity vs. shear rate of UCON-39 lubricant at different concen-trations (c = 100%, 90%, 70% and 60%) and using stainless steel (SS) and polycarbonate (PC) plates. Filled symbols are results using polycarbonate plate. Open ones are results using steel plate. At concentrations less than 100%, the apparent viscosity is lower for the PC plate than the SS plate . . . . 108

Figure A.1An example of analyzing the wear test by AFM soft ware. . . 140

Figure A.2Scanning wear test on polyethylene (MW: 900) sample. AFM imagine. Load: 10 µN, scan size: 2 µm × 2 µm. From front left to back right, 1 to 6 passes. . . 141

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LIST OF TABLES

Table 2.1 Tips used in the study . . . 8

Table 3.1 The UCON series lubricants . . . 24

Table 3.2 The Pluronic series lubricants . . . 25

Table 4.1 Comparison between normal and pathological synovial fluid . . . 44

Table 4.2 Summary of synovial joints lubrication mechanisms . . . 60

Table 5.1 Experimental solution . . . 82

Table B.1 Coefficient of friction vs. load of polyethylene (PE9, MW: 900, PE18, MW: 1800) and polypropylene (PP) using Con-50 tip . . . 142

Table B.2 Coefficient of friction vs. load of polyethylene (PE9, MW: 900, PE18, MW: 1,800) using Berk-50nm tip . . . 143

Table B.3 Coefficient of friction vs. load of polyethylene (MW: 5,600), cellulose and polypropylene using fCON-100 tip . . . 143

Table B.4 Coefficient of friction vs. load of polypropylene with lubricants (1 vol%) using fCON-100 tip . . . 144

Table B.5 Coefficient of friction vs. load of polyethylene with lubricants (1 vol%) using fCON-100 tip . . . 144

Table B.6 Coefficient of friction vs. load of cellulose with lubricants (1 vol%) using fCON-100 tip . . . 145

Table B.7 Coefficient of friction vs. load of polyethylene using HA (MW: 2 MDa) in PBS as lubricants using fCON-100 tip . . . 145

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Table B.9 Coefficient of friction vs. load of polyethylene using albumin (bovine serum albumin, BSA 11 mg/mL) and γ-globulins (γ-G) of different

concen-trations in PBS as lubricants using fCON-100 tip . . . 146

Table B.10Coefficient of friction vs. load of polyethylene using albumin (bovine serum albumin, BSA 11 mg/mL), HA (MW: 2 MDa, 3 mg/mL) and γ -globulins (γ-G) of different concentrations in PBS as lubricants using fCON-100 tip . . . 147

Table B.11Coefficient of friction vs. load of polyethylene using DPPC (dipalmitoyl phosphatidylcholine) of different concentrations (in PG)and DPPC with HA (MW: 2 MDa, 3 mg/mL) (in PBS) as lubricants using fCON-100 tip . . . 147

Table B.12Time effect on η0 (Pa s) of NaHA with D-Penicillamine . . . 148

Table B.13Specific viscosity versus concentrations of UCONs . . . 148

Table B.14Specific viscosity versus concentrations of Pluronics . . . 148

Table B.15η0 (Pa s) of UCON-12 at different concentrations using stainless steel (SS) and polycarbonate (PC) plates. . . 149

Table B.16η0 (Pa s) of UCON-16 at different concentrations using stainless steel (SS) and polycarbonate (PC) plates. . . 149

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Chapter 1 Overview

Lubrication is a very important concept in many fields, such as auto industry [1, 2], biology [3–5] and textile processing [6]. In textile processing (weaving), a lubricant would be applied to fibers to get low friction between fiber surfaces. In such way, fibers can be processed in high speed with a minimum of abrasion and fiber breakage. With the precision components with ultra-smooth surfaces being now used fre-quently in many high-tech and micro devises, such as micro-electromechanical devise (MEMS) the thickness of lubricant film has decreased continuously [7]. The distance between the sliding surfaces may reach nanometer scales, or become comparable to the dimension of the lubricant molecules. However, tribological issues still remain in such micro devices, and can cause device failure [8]. As a result, investigation of this thin film lubrication becomes more and more important with respect to the successful deployment of such high-tech and micro devices.

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synovial fluid and its components may be the way to answer these questions.

To study the tribological properties of material, different instruments can be used, such as atomic force microscope (AFM) [9–12], nanoindenter-based scratch test or scanning force microscope (SFM) [13, 14], surface forces apparatus (SFA), and clas-sical pin-on disk (POD) tribometer [15, 16]. Generally, the AFM is used to apply loads in the nanoNewton range with silicon or silicon nitride tips with a radius of curvature of 20-100 nm. AFM studies tribology at the nanoscale. The SFM uses diamond tips in intermediate radius of curvature and applies loads in microNewton range. This instrument is generally a bridge between nanotribology and traditional tribology, and works well at the microscale. Even though POD can apply loads in the miliNewton range, it is used mostly in macro-scale measurements [17]. As the dimen-sions of components and loads used decreased, scratching on the micro- to nanoscale has become very important [13]. By applying different loading ranges and contact areas obtained with these three different instruments may result in different friction mechanisms [17].

Nanoindentation is a widely used technique for measuring mechanical properties at very small scales. It is a unique tool for probing the micro/nanomenchanical prop-erties of complex material systems, such as hardness, elastic modulus and adhesion [18–20], and is able to quantify material and interfacial properties at the submicron level [21]. It can also provide a method of getting the scratching resistance of the film by nanoscratch testing [22].

In my thesis, nanoindenter based scratch tests were used to study the the lubri-cating properties of synthetic and natural lubricant solutions (synovial fluid and its components).

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Chapter 2 Tribological Study of Polymer

Films: Nanoindenter-based Scratch

Tests

2.1 Introduction

A Hystron TriboIndenter with an integrated atomic force microscope (AFM) was used in our research. It is built on a single platform designed to support numerous quantitative nanomechanical characterizations, such as elastic modulus, hardness, and coefficient of friction. The Triboindenter incorporates a three-plate capacitive transducer technology known for its sensitivity and stability. The versatile design of the Triboindenter platform has enabled the range of testing to be extended from fundamental nanoscale testing to true microindentation and microscratching.

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Figure 2.1: Picture of the nanoindenter instrument.

scanner, transducer assembly, vibration isolation system, acoustic enclosure, elec-tronic rack and peripherals, and computer and data acquisition system (See Figure 2.1.

In the instrument, there are three testing modes, quasistatic testing, nanoscratch testing and scanning wear testing. In quasistatic testing, we can determine Young’s modulus, hardness, fracture toughness and other mechanical properties by inden-tation. In nanoscratching testing, we can get quantify scratch resistance, critical delamination forces, friction coefficient and more with true nanoscale normal and lat-eral force and displacement monitoring. In scanning wear test, we can observe and quantify volumes and wear rates using the in-situ SPM imaging capability. In this study, the nanoscratch test is used.

2.1.1 Nanoscratch test

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Figure 2.2: Schematic of 2D (lateral force) Transducer.

pattern, many different types of tests can be performed. In nanoscratch testing, we can quantify scratch resistance, critical delamination forces, and friction coefficient with nanoscale normal and lateral force and displacement monitoring. The nano-scratch testing uses diamond tips. The radius of curvature of the tips can be from 50 nm to 100 µm and the applied loads are in microNewton range.

Lateral forces and displacements using a 2D transducer system are applied and measured. The transducer has three force-displacement sensors to monitor and con-trol the position in the lateral direction. As shown in Figure 2.2. The forces are applied electrostatically and the displacements are measured with a different capaci-tor technique.

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In this study the tribological properties of polymers films and thin fluid film lubrication were studied using nanoscratch tests. In most early friction studies of polymers by nanoscratching test, linearly increasing loads were applied during one scratch process [14, 20, 27], and the coefficient of friction was determined as the average of the data measured under such increasing loads. Because the coefficient of friction was found to be changed with loading, and in order to be analogous to our co-worker’s nanotribological study by AFM, in our research, a constant load was used during a scratch, and different loads were employed. The coefficient of friction was investigated as a function of the load. The reliability of the nanoscratch technique and the effect of molecular structure and fluid lubricants on friction of polymer were studied.

2.2 Experimental procedures

2.2.1 Specimen preparation

All silica wafers (WafeWold Inc.) were cleaned with Piranha solution (70% H2SO4 + 30% H2O2) for 1h and then polished by UVO (Ultraviolet Ozone) for 10 minutes immediately before spin-coating.

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smooth films. The cellulose-coated substrate was removed from the coater. Then transferred into a beaker filled with milli-Q water immerse for more than 4h and then placed in an oven. The temperature was maintained at 80 ◦C for 2h. The cellulose-coated substrate was then washed thoroughly with milli-Q water, dried with nitrogen jet and stored at room temperature in a clean chamber for further use.

Polyethylene (PE, Scientific Polymer Products, Inc., MW: 900, 1,800 and 6,500) and polypropylene (PP, Sigma-Aldrich, MW: 127,000) thin films were prepared from 0.2% solution. To dissolve the polymers, 20 mg PP/PE and 10 ml xylene (HPLC grade, Fisher) were put into a small flask with a condenser. The solution was refluxed for >2h to achieve dissolution. A wafer was placed on the stand of the spin coater, and then infrared light was used to heat to ca. 85◦C. Then one to two drops of the hot PP/PE solutions was immediately transferred onto the substrates and spun at 3000 rpm for 20 seconds. The coated substrate was then removed from the coater, and transferred into an oven. Remain at 80 ◦C for 2h to remove the residue solvent. The finished substrate was then stored at room temperature in a clean chamber for further use.

From surface color (blue) the polymer films are estimated to be ca. 100-200 nm thick. Every specimen was cut as ca. 1.5 cm × 1.5 cm small pieces from 4 inches silicon wafers.

2.2.2 Nanoscratch test

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Table 2.1: Tips used in the study Tip Name Commercial

Name

Radius Geometry Material Work for

Berk-50nm ti-039 50 nm Three-sided Pyramidal

Diamond Dry Surface

Con-50 ti-045 50µm Conical Diamond Dry Surface fCon-100 ti-077 100 µm Conical Diamond Fluid and Dry Surface

longitudinal and lateral directions, and 13 nm in the normal direction.

The scratch speed was 0.33 µm/s, and the scratch length was 10 µm. For each sample, loads of 5, 7, 10, 15, 20, 30, 40, 50, 60, 80 and 100 µN were chosen for the scratch tests. Each scratch test was controlled at one constant loading, and was repeated three times on the same surface. All scratch tests were repeated on another surface of same polymer to verify the data’s reproducibility.

For tests on polymer surfaces, a quick approach is done so the instrument knows where the surface is. After the quick approach, the normal load is defined. After the tip loads on the surface, the test is performed. However, for a test with a fluid film, the quick approach must be done first, and then several drops of solution can be placed on the polymer surface. Then the normal load is defined, the tip will go through the solution and load on polymer surface, and perform the test. If the quick approach is not done before placing the liquid on the surface, the instrument may think the surface of the droplet is the polymer film.

The nanoindentation tips used in this study are shown in Table 2.1.

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2.2.3 Data analysis

For each scratch test, ca. 1000 data points were recorded by the instrument. The normal force and lateral force were measured at every data point. The coefficient of friction was calculated by using equation:

Coefficient of friction = Lateral force/Normal force (2.1)

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2.3 Results and discussion

2.3.1 AFM images of scratch tests

Figure 2.4 shows the effect of scratching at different loadings by two different tips on polyethylene surfaces. As the loading increases, the depth of the scratch increases. That means, at high loading, the tip would deform polyethylene surface, and the higher the loading, the greater the deformation of the surface. This process yields different types of interactions between surface and tip. Generally, at very low contact pressures, elastic forces are dominant and surface adhesion plays a significant role. However, at high contact pressures the hard tip penetrates the polymer film and, as a result, the contribution of the plowing friction mechanism to the overall friction increases [17]. In Figure 2.4, the plowing of the tip on polymer surface can be clearly observed under high loadings.

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Figure 2.5: Coefficient of friction vs. load of polyethylene (PE9, MW: 900, PE18, MW: 1800) using Berk-50nm and Con-50 tip. The dashed lines were the repeated experiment results of the solid lines. Different coefficient of friction measured by two different tips illustrates different friction mechanism involved in scratching process.

2.3.2 Nanoscratch test on polymer surface

As discussed above, by using different tips, different types of friction mechanisms may be involved in the scratching process, and may result in different coefficients of friction. Figure 2.5 is the plot of coefficient of friction versus load of polyethylene with different molecular weights by using different tips.

Here, each point is the average of three tests at a given load at different locations on the same sample. In order to evaluate the data reproducibility of the test and the sample preparation, the entire testing was repeated on a second sample, as shown in Figure 2.5. By using the same tip, similar coefficient of friction data were found for both samples of the same polymer. Similar results are found for both tips and both molecular weights of polyethylene.

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At low loads, the surface interactions between tip and surface are going to dominate the experiment, but at high loads when there is more deformation of the polymer surface, the low molecular weight polymer, which is like a grease may be deformed to the point where the wafer is interacting with the tip.

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Figure 2.6: Coefficient of friction vs. load of PE (PE 9, MW: 900), PE (PE18, MW: 1,800) and PP surface using Con-50. The dashed lines were the repeated experimental results of the solid lines. Different coefficient of friction between PE and PP shows that different microstructure may give rise to different sliding resistance.

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2.3.3 Thin fluid film lubrication study

Lubrication is a very important concept in many fields, such as auto industry [1, 2], biology [3–5] and textile processing [6]. In textile processing, a lubricant would be applied to fibers to get low friction between fiber surfaces. In such way, fibers can be processed in high speed with a minimum of abrasion and fiber breakage. With the precision components with ultra-smooth surfaces being now used frequently in many high-tech and micro devises, the thickness of lubricant film has decreased continuously [7]. The distance between the sliding surfaces may reach nanometer scales, or become comparable to the dimension of the lubricant molecules. By using nanoscratching test we can achieve this thin fluid film and study lubrication at this scale.

fCon-100 is a conical fluid cell tip (radius: 100µm). It can be used to do scratching test on polymer film surface with fluid present. In our experiments, this tip was used to do scratch tests on polyethylene, polypropylene and cellulose surfaces with and without water present on the surface. The tribo-pair is a diamond/polymer.

Figure 2.7 a) shows us the steady-state coefficient of friction as a function of normal load for polyethylene, polypropylene and cellulose. Within the load range, the measured coefficient of friction decreases with increasing loads. Similar trends for polymer surfaces were showed in previous studies [20, 27, 29]. Mailhotet al. explained that this may be because of the effect of surface roughness. At high loads, because the tip can penetrate the surface and plow through the surface, the roughness effect on the friction become secondary. However, under low loads, the tip just slides along the surface topography and the roughness effect on the friction become significant [29].

Cellulose shows low coefficient of friction, similar to polyethylene. The reason for this may come from its structure. Cellulose is a straight chain polymer, and the molecule adopts an extended and stiff rod-like conformation. The multiple hydroxyl groups on the glucose residues from one chain form hydrogen bonds with oxygen molecules on another chain, holding the chains firmly together side-by-side. This compact structure may make it has low resistance to the sliding.

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(water contact angle 28.6◦) surface. If two tribopairs are both hydrophobic, hydropho-bic adhesion would occur and play an important role in surface frictional behavior [30–32]. For cellulose, there is no hydrophobic adhesion in its frictional behavior and therefore, it shows low frictional behavior.

From Figure 2.7 b), c) and d), we see water has different effect on the three differ-ent polymer surfaces. For polypropylene, after using water as lubricant, its coefficidiffer-ent of friction decreased. However, for polyethylene and cellulose, water has no effect on their friction. Soft elastohydrodynamic lubrication occurs when the contact involves an elastomer. In this lubrication regime, the soft tribopair would deform under pres-sure forming a thin fluid film between two surfaces. To study this thin fluid film lubrication, the predicted film-thickness, which is determined by the deformation of soft tribopair, and surface roughness are generally used to evaluate the effectiveness of the fluid film lubricating properties. The ratio Λ of the calculated elastohydro-dynamic minimum film thickness to some measure of the initial composite surface roughness of the actual surfaces is used to see if a fluid film lubrication occurs [32– 35]. Since polypropylene and polyethylene are both soft materials, and the radius of the diamond tip is relatively big (about 100µm), soft elastohydrodynamic lubrication is possible. However, roughness is also very critical for the fluid film formation on polymer surfaces. According to our co-workers study, roughness for polypropylene and polyethylene are ca. 0.7-2.3 nm and 4-6 nm [36], respectively. As a result, if they have similar predicted fluid film thickness, elastohydrodynamic lubrication may occur on polypropylene, but not on polyethylene. It may account for the different lubricating properties of water for polypropylene and polyethylene.

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hydrophilic PDMS. By using AFM, our co-workers also found that water decreases the coefficient of friction of polypropylene surface [39].

Based on our experiments, we think water has lubricating properties for hydropho-bic surfaces as long as it can form a fluid film between the tip and surfaces. The different lubricating behavior of water for polypropylene and polyethylene arise from their different roughness. Since polypropylene has low roughness, water forms a fluid film, and as a result, it lubricates polypropylene surfaces. However, because of high roughness of its surface, water can not form a fluid film on polyethylene, and therefore, dry contact occurs between tip and its surface.

Earlier studies have shown that moisture and temperature also affect tribological properties of surfaces, especially hydrophilic surfaces. Study by Sherge et al. has shown that humidity and temperature have strong effects on the friction of silicon. With a increase of temperature, the friction of hydrophilic silicon surface decreases, as well as the friction dependence on humidity. On the other hand, with a increase of humility, the friction of hydrophilic silicon surface increases. In contrast, hydrophobic silicon shows a weak dependence on humility changes [40]. Our experiments were done under ambient conditions and humidity was not actively controlled. Our cellulose surfaces may have absorbed some moisture from the air, which may in turn result in the negligible effect of water on measured tribological properties of cellulose.

As we discussed, surface properties, such as hydrophobic or hydrophilic, modulus and roughness, as well as the moisture and temperature all affect the measured coef-ficients of friction. Measurements on same polymer surfaces with different roughness or different testing environment may result in different measured coefficients of fric-tion. As a result, controlling the samples surface properties and testing environment is very critical for our tribological and lubricating studies.

2.4 Conclusions

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are very reliable. On the other hand, the different friction results measured by two different kinds of tips (different shape and different radius) tell us that the measured friction is related to contact area and contact pressure. Generally, coefficient of friction measured by smaller radius tip is higher than the ones measured by bigger tip, due to its smaller contact area and higher contact pressure.

Different polymer surfaces also result different coefficient of friction. In addition to the contact area and contact pressure, the microstructure, hydrophobicity and semi-crystalline morphologies of polymeric materials may also affect their friction. In our study, polypropylene shows higher friction compared with polyethylene and cellulose. Polyethylene molecule contains linear and flexible backbone chains, as a result, it may offer less resistance to the sliding process and hence it has resulted in low friction. Cellulose also has straight chains. The compact structure makes it has low resistance to the sliding. Furthermore, its hydrophilic surface makes it have no hydrophobic adhesion in its frictional behavior. Therefore, cellulose shows low coefficients of friction. However, polypropylene has CH3- side groups which may give it high sliding resistance, and hydrophobic surfaces, which may give it higher hydrophonic adhesion, therefore, it shows high frictional behavior.

By using fluid tip, we can also test the lubricating properties of thin fluid film. Depending on the surface properties (hydrophilic and hydrophobic), modulus and roughness, water has different lubricating property for polypropylene, polyethylene and cellulose surface. As long as water can form a fluid film between tip and surfaces, water lubricates hydrophobic surfaces.

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Chapter 3 Investigation of Boundary

Lubricating Properties of Synthetic

Lubricants

3.1 Introduction

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Figure 3.1: General frictional behavior of liquid-lubricated textile yarns.

In boundary regime, an extremely thin layer, perhaps one molecule thick, between the bearing surfaces holds the surfaces slightly apart, reduces adhesion, and keeps the surfaces from interlocking [41]. Boundary lubrication occurs at low speeds and high contact pressures [44]. It depends on the lubricant film layer physically adsorbed or the chemical layer formed on contact. Therefore, understanding the properties of adsorbed layers, the chemistry and dynamics of the interfacial region between the three surfaces in the presence of a lubricant are of great importance.

With precision components with ultra-smooth surfaces now being used frequently in many high-tech and micro devises, the thicknesses of lubricant films have decreased continuously [7]. The distance between the sliding surfaces may reach nanometer scales, or become comparable to the dimension of the lubricant molecules. As a result, study of this thin film lubrication becomes more and more important. By using a nanoindenter based scratch test in the presence of a fluid film, this thin fluid film and lubrication can be studied.

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tips with a radius of curvature of 20-100 nm effectively probing tribology at the nanoscale. Even though POD can apply loads in the miliNewton range, it is used mostly in macro-scale measurements [17].

By applying different loading ranges and contact areas with these different instru-ments, different friction mechanism can result [17]. The nanoindenter-based scratch test is a bridge between nano-tribology and macro-tribology, and works well in mi-croscale. As the dimensions of components and loads used are decreased, tribological study on the micro- to nanoscale has become very important [13].

Surface forces apparatus (SFA) also has been widely used for evaluating thin film lubrication at the micro/nano-scale. It can measure film thickness and friction force between two curved mica surfaces, whose radius is ca. 2 cm. Single asperity contact is also possible, because of the molecular smoothness of cleaved mica surfaces [38, 45, 46]. During the measurements, the normal load pressing the sheets of mica together causes contact area of each sheet to deform. Therefore, a locally flattened contact area in which the sheets of mica were parallel to is prodeced[38]. The distance of two mica sheet can be controlled to produce different lubrication regime [38, 45, 46]. However, based on what described about SFA, it is not suitable for measuring the tribological properties of different polymer surfaces, and also could not measure the lubricating properties of lubricants to different polymer surfaces, which is more concerned in practical field.

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Figure 3.2: The molecular structure of UCON series

Table 3.1: The UCON series lubricants Sample name Commercial name MW n m Density

20◦C

η0 of 1% in H2O (cP)

UCON-12 50-HB-400 1230 10 13 1.041 1.25

UCON-16 50-HB-660 1590 13 17 1.051 1.45

UCON-39 50-HB-5100 3930 33 44 1.056 1.381

3.2 Experimental procedures

3.2.1 Specimen preparation

Polymeric surfaces

Polymer films were prepared as described in Section 2.2.1

Lubricant solutions

Six different lubricants were studied, including the UCON series (DOW, the chem-ical company) and Pluronic series (BASF, the chemchem-ical company). All scratch tests were performed with a 1% (volume percent) aqueous lubricant solution.

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Figure 3.3: The molecular structure of Pluronic series

Table 3.2: The Pluronic series lubricants Sample name Commercial name MW n m Density

20◦C

η0 of 1% in H2O (cP)

Pluronic-84 F68 8400 29 76 1.06 1.42

Pluronic-34 P65 3400 29 19 1.06 1.3

Pluronic-65 P105 6500 56 37 1.05 1.367

3.2.2 Nanoscratch test

The coefficient of friction of polymer films were measured using a constant loading nanoscratch technique with a TI 900 TriboIndenter with integrated AFM (Hysitron, Inc., Minneapolis, MN). A diamond tip (fCon-100, conical, radus: 100µm) was drawn over the sample surface. The instrument monitors and records the normal and lateral load and displacement of the scratch tip during scratching. The resolution of normal load is ca. 1 nN, and normal displacement resolution is ca. 0.04 nm. The lateral force resolution is 3 µN and the lateral displacement resolution is 4 nm. An optical microscope with a magnification of 10× was used to locate the scratch position on the polymer surface. The nanoindenter can make scratches at the selected position with a resolution of about 50 nm in the longitudinal and lateral directions, and 13 nm in the normal direction.

The scratch speed was 0.33 µm/s, and the scratch length was 10 µm. For each sample, loads of 5, 7, 10, 15, 20, 30, 40, 50, 60, 80 and 100 µN were chosen for scratching tests. Each scratch test was controlled at one constant loading, and was repeated three times on the same surface to verify the data’s reproducibility. The data are analyzed as an average of these tests.

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applied. After the tip loads on the surface, the test is performed. However, for tests with a fluid film, the quick approach must be done first, and then several drops of solution can be placed on the polymer surface. Then the normal load is defined, the tip will go through the solution and load on polymer surface, and perform the test. If the quick approach is not done before placing the liquid on the surface, the instrument may think the surface of the droplet is the polymer film.

3.2.3 Rheological test

The StressTech HR (ATS RheoSystems, Bordentown, NJ), a stress controlled rheometer, was used to obtain the zero-shear rate viscosities (η0) of all of the lubricant solutions. A custom made double-gap, concentric cylinder geometry was used at a temperature of 25◦C ± 0.1◦C. Generally, the concentric cylinder geometry is used with polymer solutions below 1,000 cP. The outer radius of the bob measures 26.22 mm, with an inner radius of 21.60 mm. A volume of 2.83 mL is required for accurate measurements.

3.2.4 Data analysis

Data analysis was performed as described in Chapter 2

3.3 Results and discussion

3.3.1 Results

Friction tests on polymeric film with and without water as lubricant (con-trol sample)

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Friction tests on polypropylene with lubricants

The coefficients of friction versus load of a polypropylene surface using UCON lubricants are shown in Figure 3.4 a). These three 1% UCON lubricant solutions and water all have similar lubricity for polypropylene. In contrast, with Pluronic-based lubricants on polypropylene, the results are dependent upon the lubricant, as shown in Figure 3.4 b). In this case, 84 has slightly better lubricity than Pluronic-65, and both have better lubricity than Pluronic-34. Compared to the UCON series, the Pluronic series have better lubricity for polypropylene.

Because of the different adsorption energies, lubricants should show different lu-bricity for polypropylene. However, the lulu-bricity of three UCON lubricants are nearly identical (only a little better than water), so we propose that the solvent (water) has a greater effect on the lubricity of the lubricant solution. It perhaps forms a layer of molecules between the tip and the surface.

If the structure of the lubricant is changed from di-block to tri-block, the lubricity improves. The solvent doesn’t dominate the lubricity anymore. All of the Pluronic based lubricants show better lubricity than water. The reason for this may because Pluronic series have higher molecular weights than UCON series. When the molecular weight is high enough, lubricant may have stronger effect on polypropylene than water.

Pluronic-84 and Pluronic-65 have better lubricity than Pluronic-34 as shown in Figure 3.4 b). The reason for this may because of PEO groups. Compared to Pluronic-34, Pluronic-84 has same formula weight of PPO block, but longer PEO block. The result suggests that with the same PPO block, the longer PEO block, the better lubricity. The PEO block may work as a surface protection brush for polypropylene.

Friction tests on polyethylene with lubricants

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All of them show better lubricity than water, and with different structures, they show different lubricity. As shown in Figure 3.5 a), UCON-16 has similar lubricity as UCON-12 and both of them have better lubricity than UCON-39. The UCON series lubricants show different lubricity for polypropylene from polyethylene. The difference may arise from the solvent (water). As shown in Figure 2.7 b) and c), solvent (water) can affect their lubricity differently. If the solvent effect is greater than lubricant, solvent may dominate the lubricity (as for polypropylene), otherwise, lubricants would dominate the lubricity of solutions (as for polyethylene). Compared to water, all lubricant show much better lubricating properties.

Again, Pluronic-84 and Pluronic-65 both show better lubricity than Pluronic-34 as shown in Figure 3.5 b). The reason for this may also because of PEO groups. Compared to Pluronic-34, Pluronic-84 has same formula weight of PPO block, but longer PEO block. The result again suggests that with the same PPO block, the longer PEO block, the better lubricity.

Friction tests on cellulose with lubricants

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3.3.2 Discussion

As mentioned in the introduction, the lubricity of the lubricant depends on the lubricant film layer physically adsorbed or the chemical layer formed on contact with the surface. The different friction results of these lubricants may come from their different adsorption to surfaces.

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Figure 3.7: Illustration of lubricant molecule on hydrophobic polymer surface. The hydrophobic parts (PPO) adhere to polymer surface and hydrophilic parts (PEO) extend into the solution, and the more PPO groups, the stronger of the adsorption.

3.4 Conclusions

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lubricating properties of lubricants. With higher formula weight and adsorbed PPO block, the adsorption of lubricants to hydrophobic surfaces is higher, and as a re-sult, the better the lubricity of lubricants. However, too long PPO block also have an opposite effect on lubricants lubricating properties. Longer PEO block signifi-cantly contributes to the better lubricating properties of lubricants. On the other hand, longer PEO block can also decrease the adsorption of PPO groups, and as a result, decrease the lubricity of lubricants. Brush like PEO blocks work as a pro-tection for hydrophobic surfaces. UCONs and Pluronics work as effective lubricants for hydrophobic surfaces by the removal of hydrophobic interaction between tip and surfaces.

Because of none of the lubricants studied show adsorption onto cellulose surface, none of them show lubricating properties to cellulose surface. The hydrophilic surface of cellulose makes it have stronger adsorption to water than to lubricants. Therefore, water works as lubricants instead of UCONs and Pluronics.

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Chapter 4 Synovial Joint Lubrication

Introduction

4.1 Overview

Normal synovial joints (freely moving joints) provide essentially frictionless motion between limb segments, and have extremely low friction coefficients, approximately equal to an ice skate on ice [52]. Their cartilage does not abrade over several decades [42]. While this is due in part to the inherent ability of biological systems to self-maintain and self-repair, the effective lubrication of these joints also plays a significant role in their longevity [5]. Synovial joints operate well under a wide range of conditions from low loads and high shear rates to high loads and low shear rates. They also work well at start up and under impact [53].

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from a decline in both the molecular weight and concentration of hyaluronic acid (HA) [54]. Protein constituents and pH values in synovial fluid with OA and rheumatoid arthritis (RA) differ from normal synovial fluid [55].

The mechanism by which animal joints are lubricated, has evoked considerable interest for several decades. Understanding the mechanism of good lubrication in joints has obvious applications for treating arthritis and other disorders in which the bio-lubrication system has broken down, leading to cartilage and bone damage [56]. Nearly 70 million Americans, one out of every three adults, are affected by arthritis or chronic joint symptoms. As the population ages, this number will continue to grow dramatically. Arthritis is already the leading cause of disability in the United States [57]. Moreover, the costs of the disease are enormous. In 1995, the cost of medical care alone was nearly 22 billion dollars. The total costs, including medical care and loss of productivity, exceeded 82 billion dollars in 1995 [58] . While these numbers are staggering, no one can place a dollar value on the loss of quality of life, and the pain and suffering associated with arthritis. Arthritis comprises over 100 different diseases and conditions, of which the most common are osteoarthritis (OA) and rheumatoid arthritis (RA) [57], affecting ca. 21 million and 2.1 million Americans respectively [59].

Understanding the mechanism of lubrication of synovial joints may lead to the possibility of applying the mechanism at work in synovial joints to man made bearings to produce a bearing with a greater longevity [53]. Although synovial joints are subjected to more extremes of loading than any man made bearing, their lifespan is much greater than is expected of even the most efficient man made bearing [53].

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from a loaded junction or is it pressed into the cartilage? And, most important of all, what are the causes and stages of progressive degeneration [60]? Lubrication in moving joints may be controlled by the mechanical or surface properties of articular cartilage [4], or the macromolecules of synovial fluid [3, 4, 53, 56, 61–64], or some complex combination of these. In this review, we will analyze synovial fluid, artic-ular cartilage and their roles in synovial joint lubrication. And, we will summarize all mechanisms of synovial joints lubricating proposed by researchers. The potential role of hyaluronic acid, proteins and phospholipids in synovial joint lubrication and elements that affect the lubrication property in synovial fluid will be also analyzed.

4.2 Synovial joint

Synovial or diarthroidal joints are those that move freely and contain synovial fluid, such as the knee and elbow joints. The whole diarthrosis joint is contained by a ligamentous sac, the joint capsule or articular capsule (as shown in Figure 4.1 [65]). The joint capsule is aided by ligaments and the surrounding mesculotendinous tissue in holding the two bones of the joint together (e.g., stabilizing the joint). The surface of the two bones at the joint are covered in articular cartilage. The purpose of this tissue is to provide a suitable surface for lubrication and wear prevention.

In joints, where the two surfaces do not fit snugly together, a meniscus or multiple folds of fibro-cartilage within the joint correct the fit, ensuring stability and the optimal distribution of load forces.

The synovium is a membrane that covers all the non-cartilagious surfaces within the joint capsule. It secretes synovial fluid into the joint, which nourishes and lubri-cates the articular cartilage. The synovium is separated from the capsule by a layer of cellular tissue that contains blood vessels and nerves.

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Figure 4.1: The structure of a synovial joint.

[42, 66].

4.2.1 Synovial fluid

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dioxide) are too small to be detected. In some cases, such as septic arthritis, however, the synovial microvascular supply is unable to meet local metabolic demand, and significant gradients develop [65].

Synovial fluid is also crucial to synovial joint lubrication and bearing functions [5, 41, 43, 56, 68]. Synovial fluid contributes significant stabilizing effects as an adhesive seal that freely permits sliding motion between cartilaginous surfaces while effectively resisting distracting forces [65]. It has been shown that synovial fluid has a low coefficient of friction (µ)) and affords wear protection to cartilage [43, 68].

The most abundant macromolecules in synovial fluid are the sodium salt of hyaluronic acid (NaHA,∼3 mg/mL, a high molecular weight anionic polysaccharide, ∼2 MDa) and blood plasma proteins (albumin, ∼ 11 mg/mL and globulins, ∼ 7 mg/mL) [5]. Phospholipids (∼0.1 mg/mL) are also present in synovial fluid [69]. The pH values of normal synovial fluid are between 7.3 and 7.43 [70].

In early studies of synovial fluid and hyaluronic acid, scientists tried to isolate and separate those macromolecules, but getting nearly pure (protein-free) HA proved impossible. As a result, the plasma proteins were incorrectly assumed to bind irre-versibly to NaHA, forming a “NaHA-protein complex” [71]. However, recent evidence has demonstrated that bovine serum albumin (BSA), which is the most abundant pro-tein in bovine synovial fluid, binds to NaHA only at low pH, but not above pH = 5 [5]. This makes it highly unlikely that BSA binds to NaHA in synovial fluid at physiological conditions (pH = 7.4). Equilibrium dialysis suggests that only repul-sive interactions occur between BSA and NaHA under physiological condition. By studying the rheological properties of synovial fluid, phosphate buffer saline (PBS) and NaHA, Colby et al., suggested that in synovial fluid, the plasma proteins would aggregate together to form a tenuous polymeric network and the long NaHA chains entangle with this network [5].

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Figure 4.2: Rheopexy of the synovial fluid model.

at low shear rate and similar rheopexy was observed in bovine serum albumin (BSA), as shown in Figure 4.2 [72]. It was suggested that during shearing, the continuous protein network formed, and this network was responsible for the observed rheopexy [5, 52, 53]

However, compared to normal synovial fluid, pathological synovial fluid has dif-ferent composition, pH values and rheological properties, as summarized in Table 4.1. In osteoarthritis, the concentration of protein and phospholipids are 29-39 mg/mL and 0.2-0.3 mg/mL, respectively. In rheumatoid arthritis (RA), the concentration of protein and phospholipids are 36-54 mg/mL and 1.5-3.7 mg/mL, respectively. Both concentrations are much higher than in normal synovial fluid. In contrast, the con-centration and molecular weight of HA in OA are 0.7-1.1 mg/mL and 0.3 MDa, re-spectively. And the concentration and molecular weigh of HA in rheumatoid arthritis are 0.8-1.5 mg/mL and 0.6 MDa, respectively. Both the concentration and molecular weight are lower than that in normal synovial fluid [69]. The pH values of synovial fluid with OA and RA ranges from 7.4 to 8.1 and from 6.6 to 7.6, respectively. Com-pared to normal pH value, the pH value in OA is higher; while the pH value in RA is lower [70, 73].

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Table 4.1: Comparison between normal and pathological synovial fluid Normal Osteoarthritis (OA) Rheumatoid (RA) Molecular

weight of HA

∼2 MDa ∼0.3 MDa ∼0.6 MDa

Concentration of HA

∼3 mg/mL 0.7-1.1 mg/mL 0.8-1.5 mg/mL

Concentration of phospholipids

∼0.1 mg/mL 0.2-0.3 mg/mL 1.5-3.7 mg/mL

Concentration of proteins

∼18 mg/mL 29-39 mg/mL 36-54 mg/mL

Viscosity High Low Low

pH 7.3-7.43 7.4-8.1 6.6-7.6

different. While pathological synovial fluid also shows non-Newtonian behavior, it exhibits more nearly Newtonian behavior than fluid from healthy joints [74]. The fluid from the rheumatoid arthritis joint is more Newtonian than the osteoarthritic one [74]. Generally, the viscosity of normal synovial fluid is higher than the inflammatory one [54, 74, 75], and compared to normal, all modulus parameters were markedly decreased in patients undergoing index and revision total knee arthroplasty (TKA) [76].

4.2.2 Articular cartilage

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Figure 4.3: Schematic diagram of adult articular cartilage showing zonal structure.

structures, but cartilage is very different compared to bone. Firstly, the pore sizes are different. The void dimensions of bone are 50-300 µm across; those of cartilage are about 50 ˚A. Secondly, the solid matrix of bone is relatively macroscopic and rigid; that of cartilage is microscopic and flexible [42].

The thickness of the cartilage varies with each joint, and sometimes may be of uneven thickness. Articular cartilage is multi-layered. Histologically, adult articular cartilage consists of four zones,i.e. superficial or tangential, transitional or interme-diate, radial and calcified zones. The superficial zone, an acellular nonfibrous region is called the uppermost superficial layer (Figure 4.3) [4].

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parallel to the collagen fibers and the cell volume is at its lowest. The proteoglycan content is high and the concentration of water lowest. The collagen fibers in the middle and deep zones are generally oriented toward the articular surface in large bundles approximately 55µm across and are randomly arranged. The deepest layer is highly calcified, and anchors the articular cartilage to the bone. It is characterized by rounded chondrocytes located in uncalcified lacunae and the absence of proteoglycans. The collagen fibers are arranged perpendicular to the articular surface and anchored in a calcified matrix. The junction between calcified and uncalcified cartilage is called the tidemark and shows up as a line in stained histologic sections [4, 42, 77].

A highly viscous fine granular electron dense material covers the superficial ar-ticular cartilage. This uppermost superficial layer was reported to be 30 nm-2 µm thick [4, 78, 79]. This single continuous bio-membrane structure was successfully de-tected on articular surfaces of rat patella using a transmission electron microscope [61]. Three laminar structures on the articular surfaces, in which a hydrophobic layer is po-sitioned between two hydrophilic layers, were confirmed by gently removing cartilage fixed with osmium tetroxide immediately after sacrificing. The superficial membrane structure is supposed to affect the contact behavior such as boundary friction [80–82]. Study by P. Kumar et al. showed that it was resistant to hyaluronidase, partially digested with chondroitinase ABC (protease free) and was completely digested by alkaline protease. They suggested that the superficial layer is either glycoprotein and/or protein [4].

Articular cartilage does not have a blood supply. Rather, it gets its oxygen and nutrients from the surrounding joint fluid. When a joint is loaded, the pressure squeezes fluid, including waste products out of the cartilage and when the pressure is relieved, the fluid seeps back in together with oxygen and nutrients. Thus, the health of cartilage depends on it being used. Unfortunately, once it is injured, cartilage has a limited ability to repair itself [77].

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of the charges fixed to the solid matrix. The function of articular cartilage as weight-bearing tissue and its ability to undergo reversible deformation depend on the specific arrangement of collagen fibers into a three dimensional arranged collagen network that can balance the swelling pressure of the proteoglycan-water gel [77]. Normal cartilage is very durable and elastic providing a shock absorber for our joints [65] However, some researchers think cartilage is not a shock absorber because it is so thin that its capacity to absorb energy is insignificant compared to eccentric contractions of muscles and energy absorption in the bones on either side of the joint, and cartilage does not serve to ”cushion” or ”reduce impulsive forces in joints. Instead, cartilage provides a self-renewing, well-lubricated, load-bearing surface [42].

4.2.3 Lubrication of synovial joints

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Boundary lubrication

Researchers who support boundary lubrication believe that an extremely thin layer, perhaps one molecule thick, between the bearing surfaces holds the surface pro-jections slightly apart, reduces adhesion, and keeps the propro-jections from interlocking [41]. The primary reason that researchers believe boundary lubrication occurs in syn-ovial joints is that when anyone stands for 30 min, all fluid are going to be “squeezed out” from between the loading-bearing articular surfaces of the knee. However, unless that person has arthritis, the knee joint is still perfectly lubricated the moment he or she moves [84]. Also, synovial joint surface are not in continuous motion. Indeed, they are stationary much of the time. Synovial joints often move the slowest when carrying the highest loads, and only boundary lubrication could operate (showing remarkably low friction coefficients) for cartilage under this condition [42].

Studies have shown that the boundary lubricant not only present in synovial fluid, but also it is present in the superficial zone of cartilage [4, 61, 68, 85, 90].

However, people, who postulated that there is no boundary lubrication, pointed out that the friction coefficient of solid contact is too large to say there is lubrication [87]. As pointed out by Dintenfass in 1963, with elastic properties of synovial fluid and articular cartilage, as long as the synovial fluid is normal and as long as the articular cartilage remains elastic, the articulate surfaces are never extremely close to each other and, consequently, the boundary lubrication does not take place. However, if with pathological changes in synovial fluid, a boundary-type of lubrication may exist [91].

Several different substances have been proposed as boundary lubricants for artic-ular cartilage. However, the nature of the surfactant has not been resolved [92]. The cartilage surface is believed to play a major role in synovial joint boundary lubrication [4, 81, 93, 94]. Synovial fluid may also play a role in boundary lubrication.

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Radin and co-workers centrifuged normal synovial fluid (SF) and found that it separated into two layers, one a “hyaluronate” layer and the other a “proteinaceous” layer. The surprising result was that friction tests showed how the vital load-bearing ingredient resided in the “proteinaceous” and not the “hyaluronate” layer. They then followed the traditional biological route of searching for a protein unique to the joint, since serum does not lubricate, and found a macromolecular proteoglycan of molecular weight in the vicinity of 22,7500 g/mol. Its three-stage extraction from SF is most tedious, but it was found to impart load-bearing boundary lubrication, so they termed it “lubricin”. Water solubility of this macromolecule is derived from protein chains and carbohydrate chains [96–98]. While 13% of the “proteinaceous” material remained unidentified at that time, this is the fraction that was found to be capable of binding to articular cartilage [98, 99]. Lubricin was proved to impart the boundary lubrication in synovial fluid in many papers [42, 68, 100]. Some researchers found that the lubricating ability of rabbit SF diminished after injury, as shown by the elevation in coefficient of friction, and at the same time, lubricin concentration in SF decreased. So they claimed that deterioration of the boundary-lubricating ability of SF could result from decreased lubricin synthesis or increased degradation [101].

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As a result, some researchers think that lubricin plays an important role in joint lubrication but is not the lubricant per se [84].

The lamina splenders, which is thought to lie on the surface layer of the artic-ular cartilage, is a mono/multilayered, osmiophilic pseudomembrane. It consists of glycosaminoglycans, protein, and phospholipids [61, 80]. And lamina splenders is also believed to act as a boundary lubricant [61, 103]. Some authors hypothesized that surface-active phospholipids (SAPL) in lamina splenders plays the main role in boundary lubrication of synovial joints [61, 90, 95, 97, 103, 104]. Studies showed that SAPL could decrease the frictional coefficient of boundary lubrication and the friction coefficient decreased depending on the phospholipids film number, i.e. more film layers, lower fiction coefficient [81, 90]. A study from Hills et al. using enzyme treatment of cartilage and SF, found that damage to phospholipids in both the car-tilage and SF resulted in a loss of lubrication but that damage to HA had no effect [105]. A study by Ballantine and co-workers also proved that SAPL layer acts as a boundary lubricant and the depleting of this layer causes accelerated wear of articular cartilage [106].

Also, some studies found that additional γ-globulins significantly contributed to boundary lubrication. It is estimated that amphiphilies such asγ-globulins are found naturally in the superficial bilayers, which are highly detectable in the articular sur-faces. Non-polar residues of the globular proteins and glycoproteins are included in the biomembrane structure in which hydrophobic groups meet. The adsorbed films composed of bilayers were supposed to contribute to the superior frictional char-acteristics. Higaki proved that the Lα-dipalmitoyl phosphotidylcholine (Lα-DPPC) liposomes andγ-globulins can form protective films on the articular surfaces and keep their superior frictional characteristics compared to HA [61].

Investigation of Synthetic and Natural Lubricants

DEDICATION

BIOGRAPHY

ACKNOWLEDGEMENTS

TABLE OF CONTENTS

LIST OF FIGURES

LIST OF TABLES

Chapter 1

Overview

Chapter 2

Tribological Study of Polymer

Films: Nanoindenter-based Scratch

Tests

2.1

Introduction

2.1.1

Nanoscratch test

2.2

Experimental procedures

2.2.1

Specimen preparation

2.2.2

Nanoscratch test

2.2.3

Data analysis

2.3

Results and discussion

2.3.1

AFM images of scratch tests

2.3.2

Nanoscratch test on polymer surface

2.3.3

Thin fluid film lubrication study

2.4

Conclusions

Chapter 3

Investigation of Boundary

Lubricating Properties of Synthetic

Lubricants

3.1

Introduction

3.2

Experimental procedures

3.2.1

Specimen preparation

3.2.2

Nanoscratch test

3.2.3

Rheological test

3.2.4

Data analysis

3.3

Results and discussion

3.3.1

Results

3.3.2

Discussion

3.4

Conclusions

Chapter 4

Synovial Joint Lubrication

Introduction

4.1

Overview

4.2

Synovial joint

4.2.1

Synovial fluid

4.2.2

Articular cartilage

4.2.3

Lubrication of synovial joints