AN INVESTIGATION OF EFFECTS OF NOVEL POLYMERIC STRUCTURES ON PHYSICAL PROPERTIES OF CONVENTIONAL GLASS-IONOMER CEMENTS

THESIS

Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in the Graduate School of The Ohio State University

By

Alireza Moshaverinia, DDS

*****

The Ohio State University 2009

Dissertation Committee:

Dr. Scott R. Schricker, Adviser

Dr. William A. Brantley Approved by

Dr. William M. Johnston

Dr. Dr Sarandeep S. Huja ______

Adviser

Dentistry Graduate Program

Copyright by

Alireza Moshaverinia

2009

ABSTRACT

Glass-ionomer cements were first introduced to dentistry in late 1960’s.

They have proven to be useful in various areas of dental science, such as restorative dentistry. As aqueous polyelectrolyte systems, glass-ionomer cements have unique properties such as adhesion to moist tooth structure without any pretreatment, prolonged fluoride release which inhibits recurrent caries, acceptable aesthetics and biocompatibility, making these materials popular and desirable for restorative dentistry. However, they have some deficiencies such as poor mechanical properties and water sensitivity. Recently, there have been significant changes and modifications in the formulations of the acid and basic parts of the glass-ionomers, leading to enhanced mechanical and handling properties of the material.

The overall objective of this study was the modification of the glass- ionomer cement composition in order to enhance their mechanical and handling properties. N-vinylpyrrolidone (NVP) and N-vinylcaprolactam (NVC) were incorporated into glass-ionomer polymeric structure and the effects on mechanical and surface properties of the resulting cements were investigated.

The effects of these novel formulations on mechanical and handling properties of glass-ionomer cements were studied.

ii It was hypothesized that by increasing the degree of poly-salt bridge formations and cross-linking within the matrix of the set cement, the mechanical properties of glass-ionomer cements will be enhanced, which in turn, would make these kind of materials a proper choice for posterior tooth restoration and even as a bone grafting material in stress bearing areas. Based upon this research and previous studies, the hypothesis was not rejected. According to the current level of intensive research on the properties of the glass-ionomer cements, there is a strong hope to improve the clinically related properties of these materials.

iii

DEDICATION

This thesis is dedicated to my wife: Sahar, who has provided me with her endless love, support, and motivation.

iv ACKNOWLEDGMENTS

I would like to express my deepest gratitude to my adviser, Dr. Scott R.

Schricker for his continuous support and guidance throughout my studies and research, also for his mentorship and most of all enthusiasm throughout my project.

I would also like to thank Dr. William A. Brantley and William M.

Johnston for their invaluable support and advice. They have been always there to listen and encourage me and I am deeply grateful to them for their ubiquitous role in all aspects of my academic career.

Special thank you to Dr. Nima Roohpour for all the interesting discussions and collaborations during my studies both at QMUL and OSU.

Also, for all of the technical support, I would like to thank Mr. Karl Kipp.

Thanks for the friendship and support of my friends at OSU: Drs.

Fengyuan Zheng, Brian Crouse and T.B.S. Thirumamagal.

Many thanks to my lovely wife and my family for their unconditional love, support and encouragement, without which I would never have got this far.

v VITA

July 24, 1980……………………………Born – Mashhad, Iran

March 2004……………………………...Doctor of Dental Surgery,

Mashhad Dental School, Mashhad

University of Medical Science, IRAN

2004-2006………………………………..Private Dental Practice

2006- 2007……………………………….Graduate Teaching Assistant

(MPhil/PhD Student, Queen Mary

University of London, UK)

2007 – present……………………………Graduate Research Associate,

Section of Oral Biology,

College of Dentistry, The Ohio

State University

PUBLISHED ABSTRACTS AND PUBLICATIONS

1. A. Moshaverinia, F. Zheng, N. Roohpour, S. Ansari, and S. Schricker. Effects of N-vinylcaprolactam containing polyacids on fracture toughness of GICs. J Dent Res 2009; 88 (Special Issue A) Abstract No. 1032.

2. N. Roohpour, A. Moshaverinia, S. Qasim and I.U. Rehman. Development of a Novel Antibacterial Membrane for Guided Tissue Regeneration. J Dent Res 2009; 88 (Special Issue A) Abstract No. 3253.

vi 3. A. Moshaverinia, N. Roohpour, S. Ansari, I. U. Rehman and S. R. Schricker. Synthesis and Characterisation of a Novel N-Vinylcaprolactam (NVC) Containing Acrylic Acid Terpolymer for Applications in Glass-Ionomer Dental Cements (GIC). CERMACS 2008, Abstract No. 134. American Chemical Society, 40th Central Regional Meeting.

4. A. Moshaverinia, N. Roohpour, S. Ansari, M. Moshaverinia, I.U. Rehman and S. Schricker. Effects of N-vinylpyrrolidone containing polyelectrolytes on surface properties of GICs. J Dent Res 2008; 82 (Special Issue A) Abstract No. 203.

5. A. Moshaverinia, S. Ansari, Z. Movasaghi, R.W. Billington, JA Darr and I.U. Rehman. Mechanical Properties of GIC Modified with N-vinylpyrrolidone, Nano-hydroxy and Fluoroapatite. British Society for Dental Research NOF Joint Scientific Meeting, Abstract No. 145.

6. A. Moshaverinia, JA Darr and I.U. Rehman. 2008. Supercritical Fluid Processing. Encyclopedia of Biomaterials and Biomedical Engineering. ed. Gary L. Bowlin, Gary Wnek. 2nd, 4. London, UK: Informa Healthcare. Pages: 2522-2530.

7. A. Moshaverinia, N. Roohpour, S. Ansari, R.W. Billington, JA Darr and I.U. Rehman. Synthesis of N-vinylpyrrolidone modified acrylic acid copolymer in supercritical fluids and its application in dental glass-ionomer cements. Journal of Material Science: Materials in Medicine, 19 (2008): 2705-2711.

8. A. Moshaverinia, S. Ansari, M. Moshaverinia, N. Roohpour, J.A. Darr and I.U. Rehman. Effects of incorporation of hydroxyapatite and fluoroapatite nano- bioceramics into conventional glass-ionomer cement (GIC). Acta Biomaterialia, 4 (2008): 432-440.

9. A. Moshaverinia, S. Ansari, Z. Movasaghi, R.W. Billington, JA Darr and I.U. Rehman. The mechanical properties of conventional glass ionomer cements modified with N-vinylpyrrolidone containing polyacids, nano-hydroxyapatite and fluoroapatite. Dental Materials, 24 (2008): 1381-1390.

10. A. Moshaverinia, N. Roohpour and I.U. Rehman. Synthesis and characterisation of a novel N-vinylcaprolactam (NVC) containing acrylic acid terpolymer for applications in glass-ionomer dental cements (GIC). Acta Biomaterialia, xxx (2009) xxx–xxx.

12. A. Moshaverinia, N. Roohpour and I.U. Rehman. Synthesis and characterization of novel fast set proline derivative containing glass-ionomer

vii cement with enhanced mechanical properties. Acta Biomaterialia, 5 (2009): 498-507.

13. A. Moshaverinia, N. Roohpour, J.A. Darr and I.U. Rehman. Synthesis of a proline modified acrylic acid copolymer insupercritical CO2 for glass-ionomer dental cement applications. Acta Biomaterialia xxx (2009) xxx–xxx.

14. A. Moshaverinia , F. Zheng, S.R. Schricker and A.R. Mohammad. Review Article: Root caries in the GeriatricPopulation: Epidemiology, Etiology, Diagnosis, Treatment Planning and Modalities of Treatment. Dental Forum 2008, XXXVI (2): 63-70.

FIELDS OF STUDY

Major Field: Dentistry

Oral Biology – Biomaterials track

viii TABLE OF CONTENTS

Page

Abstract………………………………………………………………………….ii Dedication……………………………………………………………………....iv Acknowledgments………………………………………………………..……...v Vita……………………………………………………………………………...vi List of Tables…………………………………………………………………..xii List of Figures…………………………………………………………...…….xiii

Chapters:

1. Review of literature…………..………………………………………….1

1.1 Introduction………………………………………………………….1 1.2 Chemical composition of glass-ionomers…………………………...2 1.2.1 Chemical composition of GIC powder……………………2 1.2.2 Chemical composition of GIC polyacid…………………..5 1.3 Setting reaction and its mechanism in GICs………………………...8 1.3.1 Ion leaching phase…………………………………………8 1.3.2 Gelation phase……………………………………………..9 1.3.3 Maturation phase…………………………………………11 1.4 Chemical structure of set cement…………………………………..12 1.5 Mechanical properties……………………………………………...14 1.6 Biological properties of glass-ionomer cements…………………...16 1.6.1 Interfacial properties and adhesion to tooth structure……16 1.6.2 Fluoride release…………………………………………..18 1.6.3 Biocompatibility…………………………………………19 1.7 Applications of glass-ionomers…………………………………….20 1.8 Resin modified glass-ionomer cement……...…………………….21 1.8.1 Composition of RMGICs………………………………...22 1.8.2 Setting reaction of RMGICs……………………………..23 1.8.3 Characteristics of RMGICs………………………………24 1.9 Poly phosphonate cements…………………………………………25 1.10 Amino acid containing glass-ionomer cements…………………..26 1.11 N-vinylpyrrolidone (NVP)-containing GICs……………………..28 1.12 Modifications in GIC powder formulations………………………31 1.13 Summary and future aspects…………………………………….32 ix 2. Significance and Hypotheses…………………………………………..34

2.1. Significance………………………………………………………..34 2.2. Hypotheses………………………………………………………...34

3. N-vinylpyrrolidone (NVP)-containing glass-ionomer cements…….….36

3.1 Introduction………………………………………………………...36 3.2 Materials and methods……………………………………………..39 3.2.1 Materials…………………………………………………39 3.2.2 Methods…………………………………………………..39 3.2.2.1 Synthesis of NVP-containing polymer….. ….39 3.2.3 Characterization………………………………………….40 3.2.4 Formulation and evaluation of properties of GIC…….....42 3.2.4.1 Specimen preparation…………………………..42 3.2.4.2 Contact angle measurement……………………43 3.2.4.3 Bond strength measurements…………………..44 3.2.5 Statistical Analysis……………………………………….45 3.3 Results……………………………………………………………...46 3.3.1 Characterization of synthesized terpolymer……………...46 3.3.2 Contact angle measurement results………………………51 3.3.3 Bond strength properties…………………………………53 3.4 Discussion………………………………………………………….57 3.5 Conclusion…………………………………………………………61

4. N-vinylcaprolactam (NVC)-containing glass-ionomer cements…….62

4.1 Introduction………………………………………………………..62 4.2 Materials and Methods…………………………………………….62 4.2.1 Materials…………………………………………………67 4.2.2 Methods………………………………………………….67 4.2.2.1 Polymer synthesis……………………………...68 4.2.2.2 Characterization of synthesized terpolymer……68 4.2.2.3 Formulation and evaluation of glass-ionomer samples…………………………………………………………69 4.2.2.3.1 Specimen preparation………………………...70 4.2.2.3.2 Mechanical properties measurements………..70 4.2.2.3.2.1 Microhardness measurements…………...... 71 4.2.2.3.2.2 Fracture toughness measurements…………73 4.2.2.3.2.3 Flexural Strength Measurements………...... 74 4.2.2.3.2.4 Contact angle measurement………………..74 4.2.2.5 Statistical Analysis………………………...... 77 4.3 Results……………………………………………………………...77 4.3.1 Characterization of synthesized terpolymer……………...77 x 4.3.2 Evaluation of cements……………………………………85 4.4 Discussion………………………………………………………….88 4.4.1 Evaluation of cements……………………………………88 4.4.2 Surface properties of cements……………………………92 4.5 Conclusion…………………………………………………………95

References Cited………………………………………………………………97

xi LIST OF TABLES

Table Page

1.1 Composition of the powder portion of two different kinds of glass-ionomer cements (McLean et al., 1991)…………………………………………………..4

1.2 Compressive and diametral tensile strength of NVP modified commercial glass-ionomers (Yamazaki et al., 2005; Culbertson, 2001)……………………30

3.1 Yield, viscosity and molecular weight of synthesized NVP-containing terpolymer……………………………………………………………………...46

3.2 Contact angle and work of adhesion (both polar and nonpolar components) values of NVP modified glass-ionomer cement, Fuji II glass-ionomer and dentin surfaces treated with Fuji conditioner and NVP-containing polyacid (numbers in parentheses are standard deviations, numbers with same superscripts are not statistically different)…………………………………………………………..52

4.1 Reactant details for of AA-IA-NVC polymer synthesis…………………...67

4.2 Viscosity and molecular weight values of synthesized terpolymer in comparison to Fuji IX polymer………………………………………………...79

4.3 Fracture toughness values of experimental and control GIC samples after 24 hrs and one week of storage in distilled water medium at 37oC……………….85

4.4 Vickers hardness numbers (VHN) of experimental and control GIC samples after 24 hrs and one week of storage in distilled water medium at 37oC………86

4.5 Contact angles and work of adhesion (both polar and nonpolar components) values of NVC modified glass-ionomer cement, Fuji IX GP glass-ionomer and dentin surfaces treated with GC dentin conditioner and NVC-containing polyacid (numbers in parentheses are standard deviations, numbers with same superscripts are not statistically different)….………………………………….88

xii LIST OF FIGURES

Figures Page

1.1 Glass-ionomer glass powder composition (redrawn from Culbertson, 2001)………………………………………………………………………...3

1.2 Structures of different carboxylic acids monomers that are compartments at the glass-ionomers liquid(adapted from Wilson and McLean, 1988)….....6

1.3 The structure of the filler and matrix phase in set cement (Wilson and McLean, 1988)……………………………………………………………..13

1.4 The chemical structure of PVPA (polyvinyl phosphonic acid) monomer (redrawn from Ellis and Wilson, 1991)……………………………………25

1.5 Poly (AA-co-IA-co-NVP) polymer structure (Culbertson, 2001)……..29

3.1 FTIR spectrum of the synthesized NVP (N-vinylpyrrolidone)-containing acrylic acid copolymer……………………………………………………..48

3.2 DSC curve of synthesized AA-IA-NVP terpolymer showing total heat flow at a heating rate of 10 °C/min………………………………………...49

3.3 ATR-FTIR spectra of the NVP modified set GIC and Fuji II set glass- ionomer cement after one hour of setting (acid-glass neutralization)……...50

3.4 Bond strength to dentin (MPa) comparison between the NVP modified and Fuji II commercially available GIC (Results with the same superscript letters are not significantly different (p > 0.05))……………………………53

3.5 SEM images of the dentin side of fracture surface from cohesively failed NVP-containing GIC (a) and cohesively failed Fuji II (b) GIC bulk (B) and dentin structure (D) (× 1,500)…………...………………………………….55

3.6 SEM photomicrograph of surface of Fuji II GIC sample (a) and NVP- containing GIC sample (b) (× 1,500)……………………………………….56

4.1 Chemical structure of NVP (N-vinylpyrrolidone) molecule…………...63

4.2 Chemical structure of NVC (N-vinylcaprolactam) molecule…………..64 xiii

4.3 Young` s equation for calculation of interfacial surface energy………..76

4.4 Chemical structure of the synthesized terpolymer of acrylic acid-itaconic acid-N-vinylcaprolactam……………….…………………………………...78

4.5 1H-NMR spectra of synthesized NVC modified polymer……………...80

4.6 FTIR spectrum of the synthesized NVC-containing terpolymer……….81

4.7 ATR-FTIR spectra of the (a) NVC-containing terpolymer vs. Fuji IX polymer (b) NVC modified set GIC and Fuji IX set glass-ionomer cement after one hour of setting (acid-glass neutralization)………………………..83

4.8 DSC curve of synthesized AA-IA-NVC terpolymer with 8:1:1 molar ratio showing total heat flow at a heating rate of 10 °C/min……………….84

4.9 Flexural strength values for GIC samples after 24 hrs and one week of maturation in distilled water………………………………………………..87

xiv CHAPTER 1

REVIEW OF LITERATURE

1.1 Introduction

Glass-ionomer cements were invented by Wilson and Kent, at the English

Laboratory of the Government Chemist, in early 1970s [1]. Glass-ionomers are water based cements also known as polyalkenoate cements. The curing reaction is based on an acid/base reaction between its components [1, 2]. This name has been derived from the formulation of the glass powder and polymeric ionomer that contains carboxylic acids.

The nature of the set cement is an inorganic, organic network with infinite molecular weight. They are dispensed as a powder and liquid or as a powder only, where water needs to be added in recommended amounts to initiate the acid/base reaction [1-3]. GICs own certain exceptional properties that make them useful as restorative and adhesive materials, including adhesion to wet tooth structure and base metals, anticariogenic

properties due to release of fluoride, thermal compatibility with tooth structure,

translucency, biocompatibility and low cytotoxicity [1-5]. Regarding their unique

properties, GICs have many applications in dental clinics such as luting cements (type I),

filling materials (type II) and lining cements (type III) [6-9].

1 The limitations that prevent wider clinical use include brittleness, poor fracture

toughness of the material and also sensitivity to moisture in the early stages of the

placement. Many improvements have been made since their invention though GICs are

still not as durable as composite resins. Manipulating the powder to liquid ratio and the

formulation can result in many versions of glass-ionomer cements for various

applications. Stronger and more aesthetic materials with improved handling characteristics are now available, so low fracture toughness and lack of strength are still the main problems for GICs in dental applications [1-13]. It has been shown that by increasing the degree of crosslinking and through increased polysalt bridge formation,

mechanical properties improve considerably, which in turn, would make the material a

better choice for posterior tooth restoration and as a bone grafting material in stress

bearing areas [14].

1.2 Chemical composition of glass-ionomers

1.2.1 Chemical composition of the GIC powder

Glass-ionomer cements contain ion leachable fluoroaluminosilicate glass that can

react with water soluble acids such as polyacrylic acid in order to yield a cement. The

cement is the product of an acid-base reaction between the silicate glass as the basic

2 component and the poly acrylic acid homo and copolymers as the acidic component [1-

15].

Figure 1 shows the fundamental components of the glass-ionomer inorganic powder.

Similar to the powder of silicate glasses, glass-ionomer powder is a finely ground

ceramic which is soluble in acids. The main components of the powder are silica (SiO2),

alumina (Al2O3), calcium fluoride (CaF2), sodium fluoride (NaF) and cryolite (Na3AlF6) or aluminum phosphate (AlPO4). Phosphate and fluoride are used in the glass to modify

the setting characteristics. The main structure of the glass is the alumina and silica which

form the back bone and skeletal structure of the glass [15-17].

Figure 1.1: Glass-ionomer glass powder composition [14].

3 The glass structure is tetrahedral which every Si ion is attached to four oxide ions,

these Si +4 are replaced by Al +3 in the center sites with the same tetrahedral structure.

Fluoride and phosphate ions are also present in the glass structure [18, 19]. In the following table (tale 1.1), the chemical composition of the powder portion of two different kinds of glass-ionomer cements available in the market have been tabulated.

Components A (wt %) B (wt %)

SiO2 41.9 35.2

Al2O3 28.6 20.1

CaF2 15.7 20.1

NaF 9.3 3.6

AlPO4 3.8 12.0

Na3AlF6 1.6 2.4

Table 1.1: Composition of the powder portion of two different kinds of glass-ionomer

cements [1].

4

Lanthanum (La), strontium (Sr), barium (Ba) or zinc oxide has been added to

provide the radio opacity of the set cement. CaF2 has been added as a flux to decrease the

melting point (the structure can be melted at economical temperatures below 1350oC)

[18-22]. The presence of excess CaF2 induces either phase separation or crystal

precipitation within the glass and glass becomes semi transparent or opaque. Since the

fluoride ions are tightly bound to the glass structure they can easily release into the

cement matrix slowly. But the physical properties do not deteriorate even after fluoride

release [20-25].

1.2.2 Chemical composition of GIC polyacid

The acidic component of the original glass-ionomer cements consisted of an

aqueous solution of polyacrylic acid. This solution was quite viscous, unstable and tended

to gel over time. In addition to these disadvantages, the setting rate of the cement was too

low. The polyelectrolyte used in glass-ionomer cements can be described as

poly(alkenoic acids). These polyacids include the homopolymers or copolymers of

unsaturated mono, di- or tri- carboxylic acids, such as acrylic acid, maleic acid and

itaconic acid. Due to the variable number of carboxylate groups that they have, the

5 reactivity and strength of each copolymer is different from the other one [26, 27]. The

structures of these acids are shown in figure 1.2.

The role of water is often not considered, but it has an important role. Water is the reaction medium and it is important in the hydration reaction, if it is replaced by another

solvent such as alcohol, the result will be a very weak cement. The optimum amount of

water is needed for proper setting, too much water in cement system causes weakening of

the cement [17].

Figure 1.2: Structures of different carboxylic acids monomers that are compartments at

the glass-ionomer liquid [14].

6 The addition of tartaric acid to glass-ionomer cements helps makes it a practical system. The role of tartaric acid is very important, it can improve the handling properties by decreasing the setting time and increasing the working time. The viscosities of tartaric acid containing polyacrylic acid solutions do not change over the shelf life of the cement.

Tartaric acid postpones the onset of viscosity and the greater concentrations of the acid will lead to longer delays [28].

The nature of the poly acid does have some effect on cements properties; however, these effects are less than changes in powder composition. By adding itaconic acid a copolymer forms that is less strereoregular than polyacrylic acid and condensation by hydrogen bonding is hindered so this copolymer solution is stable over time and does not became gel like [29].

Copolymers of acrylic and methacrylic acids are more stable to gelation than PAA solutions but they are more viscous, causing some difficulties in the process of cement preparation. However they have good cement-forming properties. This chain stiffness also leads to hydrogen bonding and gelation prevention. These copolymers are one of the most important series in glass-ionomer cement formation and can be formed without any residual monomer.

Generally, decreasing the molecular weight of the polyacids results in a weakening of the cement. But with these copolymers, this effect is offset with higher cross linking degree by itaconic acid groups, since this acid has two COO- groups in their structure.

7 The aqueous solution of these copolymers are indefinitely stable to gelation, and this stability is because of formation of seven-member intramolecular hydrogen-bonded rings involving both types of pendant acid groups, and therefore reducing the probability of formation of intermolecular hydrogen bonding [23, 26-28].

1.3 Setting Reaction and its Mechanism in GICs

The setting reaction of glass-ionomer cement is fundamentally an acid-base reaction, but the process is complicated and not fully understood. When the powder and liquid are mixed together, a paste (hydrogel salt) forms, which is the primary result of acid-base reaction. The glass-ionomer cement sets and hardens by metal ions transfer from glass to liquid medium of acid which causes the gelation in the aqueous phase [1-8].

During transfer of these ions, the matrix is vulnerable to moisture because metal ions are in their soluble form. So there should be some protection against moisture. Another cause of early solubility is water loss, as a result of desiccation which disrupts the cement structure. Dehydration after maturation of cement is no longer a problem. The acid will attack to the powder (base) surface and the mixture becomes paste like. There are three stages or phases in the mechanism of setting of glass-ionomer cements [1-20]:

8 1.3.1 Ion leaching phase:

In initial stage of the reaction, the glass powder is decomposed by polyacid. 20%-

30% of glass particles` surfaces are degraded and the main part remains to become a core

for silica gel. Ionization of carboxylic acids produce H + ions in the solution, which then

attack the surface of glass powders and cause the release of Al+3, Ca+2, Na+, F- and

-1 H2PO4 ions which will diffuse into the solution (aqueous phase of cement) [20-22]. The

Si ions will produce silicic acid, which polymerizes at the surface of the glass powder and produce a silica gel [(Si(OH)4] n. In fact the outer surface of the glass powder dissolves in

H+ ions and first the Ca+2 ions and small number of Na+ ions are released into the liquid.

+3 +2 +1 Then Al ions will release but not in free form but in complex forms like AlF , AlF2 or other hydrated complexes. Calcium ions predominate because the acid attack on the surface is not uniform, but preferentially at calcium rich sites [30].

1.3.2 Gelation Phase

This is the intermediate stage in the setting procedure of glass-ionomer cements.

At a critical pH and ionic concentration precipitation of insoluble polyacrylates begins to take place. In a certain stage of this process setting will occur, even after the cement sets

9 the precipitation process continues until the ions are in a insoluble forms; they are

responsible for development of hardness [20-25].

The initial set begins with formation of the calcium polyacrylate salt (in the

absence of tartrate salts), but the hardening process derives from the slower formation of

aluminum polyacrylate, which ultimately predominates in the matrix. This process may

last for more than 48 hours. Not all the carboxylic acid groups (COOH) convert to

carboxylate (COO-). First, when most of carboxylic acid groups have ionized, the number

of negative charges become so great that the positively charged H+ ions become very

strongly bound to remaining unionized carboxylic groups and are not easily replaced by metal ions. Furthermore, as the density of cross-links increases the metal ions are increasingly hindered in their movements toward carboxyl sites. The concentration of the ions reaches a maximum as the glass decomposes, and then declines as the salts form. So the calcium and aluminum ions cause the polyacrylate to gel. However, Na+ ions do not

have this effect. Therefore, the calcium divalent and aluminium trivalent ions are the main cross linking ions [30, 31].

It has been reported that it is unlikely that aluminum ions connect to three carboxylate groups. The coordination number of Al3+ in water is six, therefore there must

be six ligands attached to the central aluminum ion [32, 33]. In the case of glass-ionomer

cements the ligands are water molecules, F–, OH- ions (F- and H+ are in at the same size

and easily interchangeable) and carboxylate ions (COO-) from polyacrylate.

10 +7 [Al13O4(OH)24(H2O)12] is the intermediate product that is the main cause of the delay of

releasing of Al 3+ in the solution. A low percentage of Al ions will attach to three

carboxylate groups at the same time. Metal cations are able to form complex ions in

3+ water such as [Al (H2O) 6] [29-33].

1.3.3 Maturation Phase

This is the last stage of the glass-ionomer cement’s setting reaction. Sometimes it

takes more than a year for a glass-ionomer to maturate and gain its maximum strength.

The hardening process continues for about 24 hours and is accompanied by a slight

expansion under conditions of high humidity [30]. In the first days the translucency of

cement improves and cement become resistant to dehydration. At least for a year the

strength continues to increase, proportional to the logarithm of time. The cement becomes

increasingly rigid as it ages. This is a unique characteristic of these cements among all other dental cements.

The ability of the cement to dehydrate or uptake water decreases as it matures. An

Increase in strength is related with to the amount of bound water, that is, more hydrating

of metal-carboxylate links in a manner analogous to the hydration reactions of Portland

cement. Strength of glass-ionomer cements are directly related to the bound (non-

evaporated) water/evaporated water ratio, or the degree of hydration. During the

11 maturation of the cement this ratio decreases with time, the amount of unbound or loose water becomes less and less in comparison to bound water [30, 31].

Regarding the important role of water in the initial stage of setting, glass iomomers are sensitive to either dehydration or excess moisture. So we must protect the cement from the oral environment (humid condition of the) mouth in this stage.

Desiccation will retard the reaction, and can cause shrinkage and crazing. Therefore, after setting begins and the matrix has been removed, the cement should be protected by a suitable barrier [34]. The good material for protection could be copal varnish and some other varnishes such as petroleum jelly can be effective (but they can be removed easily by tongue movement), the same work can be done by clear nail varnishes. Light curing bonding agents might be the best solution for protection [4-11].

1.4 Chemical Structure of Set Cement

According to studies of Barry et al. [30], the set cement is a highly complex composite (gel of calcium and aluminum polyacrylates that contains fluoride). Original glass powders act as filler for the cement, which is partly degraded to siliceous hydrogel that contains fluorite crystallites [35]. The filler particles are etched by polyacrylic acid and sheathed by silica gel with a moderate bond between their surfaces. Due to complete degradation, particles smaller than a certain size can not be used. The set cement has

12 hydrogel matrix, silica gel and glass particles [30-35]. There are ionic bonds between metallic ions and carboxylate groups (ionic cross linking) (figure 1.3.).

Figure 1.3: The structure of the filler and matrix phase in set cement [1].

Wasson et al. in their studies showed that there are two kinds of networks in set glass-ionomer cement, the organic network which consists of polyacid chains

(copolymers) with metallic connecting ions that make polysalt bridges [36]. The other one is the inorganic network which is based on the cooperation of the silica and phosphate ions [36, 37, 38].

13 1.5 Mechanical properties

Mechanical properties are very important in dental restorative materials [4]. In order to obtain maximum service, these materials should withstand the forces generated during mastication [4, 5]. The stability of a solid material under the applied force is determined by the nature of the material and atomic bond strengths. Materials used in restorative dentistry should have sufficient strength to withstand the complex forces that repeatedly can arise from mastication during the chewing process. The average force that is observed in chewing is about 800-1000 N [4-10].

Compressive strength is useful for comparing materials that are brittle and generally weak in tension (such as dental cements) [9]. It is less useful to determine the compressive properties of ductile materials such as gold alloys. The compressive strength of a material is defined as the maximum stress that the material can undergo without fracture. It is measured by applying a load to the flat ends of a cylinder or rectangular shaped sample [4, 5, 10]. It is necessary to adopt standard sizes and dimensions to obtain reproducible test results. Compressive strength is widely used as a method of measuring the strength of glass-ionomer cements. According to different studies, it has been shown that the compressive strength of glass-ionomer cements is in the range of 160-230 MPa

[1]. It has been reported that the compressive strength test is not a suitable testing method for inclusion as a standard technique for restorative dental materials because of the

14 variability in the results obtained [11]. It was suggested that an alternative method of evaluation should be used instead of the compressive strength testing method [4, 5].

Brittle materials must be tested with caution. During tensile strength tests tress concentration at the grips or anywhere else in the specimen can lead to premature fracture testing [5]. In other words, it is not possible to grip dumb-bell specimen without imposing surface stress resulting in fractures at the gripped end of the specimen. Thus, there has been large variability in tensile testing data on brittle materials like glass-ionomers. In an alternative testing method, diametral tensile strength, the force is applied to a short cylindrical specimen on its diameter of the cylinder [4,5,10]. Prosser et al. in their studies reported that there are some plasticities involved in the deformation of the glass ionomer cement indicating that the tensile strength that is measured by using this method may not be the best technique which can be used [39]. The compressive strength applied to the specimen introduces a tensile strength in the material perpendicular to the plane of which the force is applied. It is important to note that if the specimen deforms significantly before failure or fractures into more than two equal pieces, the data from this test may not be valid. The tensile strength that has been measured for glass-ionomer cements is about

10-40 MPa [40].

Flexural strength of a material is measured by a loading force on a simple beam, which is supported at each end. Such a test is called a three point bending test (3PB)

[4,5]. It measures the maximum tensile strength in the specimen which is related to the

15 force required to break the material. This test includes the compressive and tensile strength. This is because of the fact that compression stress exists in the upper surface of the sample, where the force is loading, and tension occurs at the lower surface of the specimen. Many brittle dental materials such as cements have tensile strength considerably lower than compressive strength [10]. Prosser et al. have reported that the most suitable test for strength measurement of glass-ionomers is the flexural test. They used flexural strength testing for glass-ionomers and concluded that it is the most practical and reliable estimation of the fracture resistance of a brittle material, such as glass-ionomer cements [39].

Fracture toughness (KIC) measures the failure of a material after continuous application of the load [10]. It represents the resistance of a material to crack propagation.

Generally, the larger the flaw is in the materials structure, the lower stress needed to cause fracture in its structure [10, 12]. Fracture toughness has been measured for wide variety of dental restorative materials including glass-ionomer cements. It has been mentioned in literature that fracture toughness values of conventional glass-ionomer can vary from 0.36 MNm-3/2 up to 0.8 MNm-3/2 [73, 98].

16

1.6 Biological properties of glass-ionomer cements

1.6.1 Interfacial properties and adhesion to tooth structure

Essential requirements for proper adhesion are good surface wetting, a clean substrate surface and low wetting angle [41]. Wilson and McLean showed that glass ionomer cements have the ability to adhere permanently to untreated enamel and dentin in moist conditions [1]. Glass-ionomers have the ability to adhere to stainless steel and tin oxide plated platinum or gold [42].

Researchers indicated that no single mechanism exists to explain the bonding mechanism of glass-ionomer cements to enamel and dentine [42]. The initial adhesion between the tooth and the fresh cement is mainly due to polar attractions and weak hydrogen bonds. A combination of mechanisms such as ionic displacement, ionic diffusion or exchange and ionic bridging are known [43]. Recent studies have shown that adhesion occurs between glass polyalkenoate cements and collagen in which a pendant

COO- groups of amino acids from collagen can be cross linked by Ca+2 from the glass.

However, previous studies stated that collagen does not adhere to glass ionomer cements.

It was indicated that polyacrylic acid is absorbed on to collagen through hydrogen bonding and that no primary bond (polar attractions) necessary for the formation of

17 polyacrylate, occur which is the main cause of weaker dentine glass-ionomer bonds. This

is the mechanism of adhesion of polyelectrolyte cement to hydroxyapatite, which is also

applied for the hydroxyapatite component of dentine [44].

Wilson et al. reported that during adsorption, polyacrylate ions entered the surface of hydroxyapatite by displacing and replacing surface phosphate groups. While the carboxylic acid groups (COO-) react with the hydroxyapatite of enamel and form ionic bonds, calcium and phosphate are displaced from the enamel. An intermediate layer forms between the cement and the surface of enamel which is rich in calcium and phosphate. The composition of this layer is different from the composition of enamel and dentine and glass-ionomer cement [45]. Causton and Johnson indicated that the glass ionomer surface contains a silicate glass in a solution of polyacrylic acid, tartaric acid,

Ca+2 and Al+3 [46].

1.6.2 Fluoride release

Glass-ionomers anti-caries effect is due to fluoride release, and the caries are

arrested at the interface of restoration–cavity wall margin. It has been shown that the

influence of the fluoride is found in a zone of resistance to demineralization which is at

least 3 mm thick around glass-ionomer restoration. Due to the ionic radius (0.136 nm) of

F- which is similar to OH groups radius (0.14 nm) and the ability of fluoride ions to

18 replace the hydroxyl group in the apaptite lattice, fluoride ions may fit better in the

hydroxyapatite lattice than the hydroxyl ions. Consequently, fluoroapatite crystals will be

formed which, regarding to their lower crystal energy; they are more durable and less

soluble in comparison to hydroxyapatite crystals. Therefore, enamel which uptakes

fluoride at hydroxyl sites is more durable to caries and to plaque acids. This is partly because of the reduced surface energy of apatite making it more difficult for plaque to adhere to enamel surfaces.

Fluoride enhances the crystallinity of the hydroxyapatite crystals and influences the composition of the bacterial plaque and its biochemistry which may alter carbohydrate metabolism in dental plaque [50, 51]. Fluoride is released from glass- ionomer cements primarily as sodium fluoride (NaF). NaF is not a matrix forming species so the cement is not weakened by loss of fluoride ions. The mechanism of fluoride release is complex and is dominated by diffusion mechanisms where the rate of fluoride release is reduced as the time passes according to concentration gradient [47].

Tay and Braden (1988) in their studies showed that fluoride release involves a two-step procedure; a rapid surface elution followed by slower continuous bulk diffusion of fluoride ions [48]. Substantial amounts of fluoride are taken up by dentin and enamel and cementum in cavity walls in contact with glass-ionomer cement and increase their resistance to acid attack indicating significant amounts of fluoride in their structure [49].

19 1.6.3 Biocompatibility

The ability of a material to act in the appropriate host response range in a specific

application is defined as biocompatibility. Since glass-ionomer cements were designed to

adhere to tooth structure and they are in direct contact with dental structures the

biocompatibility of these materials are important [52, 53]. Many in vivo and in vitro

studies have shown that in general glass-ionomer cements are biocompatible with pulpal

and gingival tissues (animal and human studies) [53].

Glass-ionomer cements are an irritant to pulpal tissue, and the amount of this

inflammatory effect depends on the residual dentine between the glass-ionomer and the roof of pulp chamber and pulp tissues. Studies stated that the inflammatory effects of the glass-ionomer cements are similar to that of zinc polycarboxylate cements less than zinc phosphate, and more than ZOE cements [53, 54].

Kawahara and his coworkers reported that, although freshly mixed paste inhibits cellular proliferation, it was not cytotoxic. However they all agreed that set cement had no adverse effect on cell cultures. The initial acidity of the unset cement may lead to pulpal necrosis because of micro leakage and subsequent bacteria penetration [53]. Large areas of dentine with thin layer and pulp should be protected by using a proper technique in order to avoid pulpal sensitivity. Lining with ZOE or Ca(OH)2 are required where less

than 1mm of sound dentine remains over the pulp [53, 54].

20 Glass-ionomer cements have a therapeutic effect. They provide excellent and durable marginal seal eliminating secondary caries, because of their continued fluoride release they also inhibit caries on adjacent tooth material. Recently due to their good biocompatibility they were used as bone grafting materials and studies have shown that glass-ionomer cements can promote bone growth [104].

1.7 Applications of glass-ionomers

Due to the direct chemical bonding between glass-ionomers and surface of tooth structure (enamel and dentin) without any additional pretreatment, glass-ionomers can be used as luting cement (type I), filling material ( type II) and lining cement (type III) in the following applications [1-20].

(1) Restoration of deciduous teeth,

(2) Restoration of erosion and abrasion lesions without cavity preparation,

(3) Fissure and pits sealant material,

(4) Restoration of class III and V carious lesions,

(5) Core build up and repair of defective margins,

(6) Minimally invasive, proximal, buccal, lingual and occlusal cavity preparation,

(7) Fixation of orthodontic appliances,

(8) Cementation of crowns, inlays and bridges,

21 (9) Lining all types of cavities,

(10) Replacement of carious dentin in sandwich technique,

(11) Sealing the root surfaces of over dentures [1-20, 55, 56].

1.8 Resin modified glass-ionomer cements

In order to overcome disadvantages of conventional glass ionmers, resin modified glass-ionomer cements (RMGICs), which are hybrid cements that cure or harden by two mechanisms, were introduced. In these cements, the system undergoes polymerization of the resin monomer while the acid-base reaction occurs simultaneously. The main attraction of this invention was the initial setting of the cement, which is obtained by photochemical polymerization and protects the cement from early moisture. RMGICs are widely used in dentistry as adhesive materials for restorations, liner, fissure sealants, retrograde treatments and bonding orthodontic brackets to tooth enamel [4-11].

1.8.1 Composition of resin modified glass-ionomer cements

The powder composition of RMGICs is the same as conventional glass-ionomer cements fluoroaluminosilicate glass containing a photo initiator (if it is dual cure) and sometimes an initiator for chemical curing (if the glass is tri-cure cement). Because this

22 photo-initiator can react with light, the glass powder should be kept inside amber colored containers and protected from light [4-10]. The liquid is a composition of aqueous polyalkenoic acid (25-45%) with a photo curable monomer, which is hydrophilic HEMA

(2-hydroxy ethyl methacrylate) (21-41%), and subsequently Bis-GMA with HEMA was added as a cosolvent. The PAA contains a small proportion of modified pendant carboxylate groups substituted with methacrylate groups as a cross linking material [57].

1.8.2 Setting reaction of RMGICs

Two kinds of setting reactions take place in RMGICs. First, the acid-base reaction between the polyacrylic acid and basic glass powder, and second, the photo polymerization which started by the oxidation/reduction reaction. The free radical polymerization reaction initiates by light curing and will result in poly HEMA formation and polymerization of pendant, unsaturated, methacrylate groups tethered to the PAA [4-

6]. In this reaction, polyHEMA copolymerizes with PAA, and as the matrix has both ionic and covalent bonds, it chemically links to polyacrylate matrix via COO- groups and

hydrogen bonding.

Infrared spectroscopy studies have shown that during the setting reaction the

double bonds in the liquid disappear after hardening and the number of carboxylic acid

groups are reduced as the acid-base reaction advances. By copolymerizing between PAA

23 functional groups and Bis-GMA and HEMA, a highly cross linked organic matrix will

result [7, 8]. These concurrent acid-base and photo polymerization reactions enhance the

physical and mechanical properties of hardened RMGICs because of the dual network

formation of both salt bridges and covalent cross linking. In tri-cure RMGICs the setting reaction takes place in absence of light curing which is called dark curing [4-11].

1.8.3 Characteristics of RMGICs

The RMGICs have the ability to bond to enamel and dentine and also they exhibited desirable amounts of fluoride release in the clinical investigations. Because of their hydrophilic components, they absorb water much more than conventional ones and they swell in aqueous medium. In comparison to conventional glass-ionomers, RMGICs have many advantages such as reduced moisture sensitivity (by incorporating photo polymerization and faster setting), extended working time, less setting time due to faster setting reaction, increased mechanical properties (higher tensile and compressive strength), enhanced translucency, desirable fluoride release (almost the same as conventional glass-ionomers) and more color stability.

However, there are still some disadvantages which need to be addressed in formulating RMGICs. These disadvantages include (1) complicated setting chemistry; (2) more shrinkage due the polymerization reaction and as a result more micro leakage; (3)

24 tendency for phase separation during the setting reaction; (4) presence of unreacted monomers (free HEMA monomers) which may be cytotoxic; (5) swelling of the polymerized and set cement because of the increased water absorbtion from saliva, food and beverages. However because of the lower water/carboxylic acid ratio of the material, the ability of wetting is reduced, which will cause micro leakage [57- 59].

1.9 Poly phosphonate cements

Polyvinyl phosphonate glass-ionomer cements first introduced to dental world in

1991 [60]. This kind of glass-ionomer is the result of reaction between special calcium aluminosilicate glass and polyvinyl phosphonic acid (PVPA) (figure 1.5), the composition of the glass is slightly different from the conventional glass-ionomers. As a result of the fact that the PVPA is a stronger acid in comparison to PAA, the PVPA salts with fluoroalumino silicate glass are less basic and due to more reactivity of PVPA; therefore, less aluminum should be used in glass powder composition. Since the higher amounts of Al in glass leads to formation of stiffer glasses, no reduction can be made in aluminum levels in the glass composition [60].

25

Figure 1.4: The chemical structure of PVPA (polyvinyl phosphonic acid) monomer [60].

In order to control the vigorous reaction between high Al containing glass and

PVPA, the glass powder was heated during the preparation procedure and some moderating additives were added to PVPA solution. For example instead of 20% CaF2 in

glass powder composition ZnF2 was used to improve cement formation [61]. The setting

reaction of poly vinyl phosphonic acid glass-ionomer cements is similar to conventional

GICs with PAA as an acid solution. The carbon double bonds in the vinyl group take part in polymerization reaction and phosphonate groups act as a network modifier for cross- linking and formation of salt bridges with metallic ions [60, 61].

26 1.10 Amino acid containing Glass-ionomer Cements

Because of the very close attachment of all the carboxylic acid groups (COOH) to the back bone of the polymer, not all the carboxylic acid groups are converted to carboxylate groups during setting reaction and formation of the salt-bridges [14].

Therefore, the introduction of monomers with various spacer lengths of the carboxylic acid groups to the polymeric backbone appears to be the other way of increasing the mechanical properties of glass-ionomers [14, 62]. Amino acids are the right choice for reaching this goal. Amino acids are biocompatible (they are the basic component of proteins in human body); in addition, acrylic functional amino acids and their derivatives have been found to be good pressure sensitive adhesives [63].

Wei et al. and Kao et al. found that by the synthesis of methacrylate or acrylate derivatives of amino acids such as glycine and β-alanine and glutamic acid, and adding the products to the liquid of glass-ionomer cements, terpolymers with AA (acrylic acid) and IA (itaconic acid) are formed [63, 54]. Thus, acrylic acid copolymers were modified with N-acryloyl or N-methacryolylamino acids, such as N acryloyl-glutamic acid, providing a possible chance for improving the glass-ionomer properties. These newly formulated polyacids have flexible side chains tethering the carboxylic acid groups at various distances from the main chain polymer backbone, allowing for more freedom and less steric hindrance when the carboxylic acid groups are undergoing chemical reactions.

27 The mechanical properties of the resulting amino acid modified glass-ionomer cements were measured and compared to conventional glass-ionomer cements [62-64].

Fracture toughness of the amino acid containing glass-ionomers was

improved. All amino acid-modified glass-ionomer cements exhibit comparable CS (193-

236 MPa) and almost four times higher FS (55-71 MPa) than the commercial product

(Fuji II, in this study) [62]. Amino acid-modified GICs show both fewer or smaller pores

and fractures on their surface. These materials showed a more integrated and highly fused

surface texture compared to Fuji II glass-ionomers [62, 63, 64].

1.11 N-vinylpyrrolidone (NVP)-containing GICs

N-vinylpyrrolidone has a number of good properties such as polymerizing with

free radical initiators, being water soluble and non ionogenic, and possessing the

properties of a synthetic polymer which is highly hydrophilic and nontoxic [65]. NVP has

the ability to form hydrogen bonds because of the presence of its amide group. NVP

homopolymer absorbs water strongly with the absorbing center of amide group. Different studies have investigated the effect of the addition of NVP to glass-ionomer cements [14,

62].

Xie et al. used the copolymer of NVP and AA with commercial Fuji II glass

ionomer and measured the mechanical properties of the samples. Results showed that in

28 order to develop new water soluble copolymers for formulating enhanced glass-ionomer

cements, copolymer of AA and NVP can be used [66]. It was found that the NVP has the

ability of improving flexural strength (from 18 MPa for commercial GIC to 20 MPa) and

diametral tensile strength as well (from 14 MPa for commercial GIC to 18 MPa) [66]. In

another study poly AA-co-IA-co-NVP (figure 1.5) were used with a 7:3:1 ratio of

monomers [14].

Figure 1.5: Poly (AA-co-IA-co-NVP) polymer structure [14].

The addition of NVP to the copolymer of the acrylic and itaconic acid; results in easier blending of the glass powder, obtaining smoother mixtures. This study showed that for achieving optimum values for FS, the molar ratio of 7:1:3 (AA/IA/NVP) is optimal, but it did not show the higher values for the CS. More NVP in the mixture produced 29 higher FS. Moreover, less NVP produced higher CS. Both of these phenomena are due to the lesser ability for salt-bridge formation with higher NVP-containing solutions, and causing less ionic cross-linking, resulting in a less brittle material [14, 62]. Yamazaki et al. synthesized the terpolymer of acrylic acid, maleic acid and NVP and used this polymer with the glass powder of the Fuji IX commercial conventional glass-ionomers

[67]. The results showed (table 3.1) that the NVP-containing polyelectrolytes are promising polymers for glass-ionomer cements with enhanced mechanical properties

[67].

Material CS(MPa) FS(MPa) DTS (MPa)

AA-co-NVP (Fuji II) 150 20 18

AA-co-IA-co-NVP (Fuji II) 276 34.0 20.5

AA-co-IA-co-NVP (Fuji IX) 277 46 21.6 Fuji IX 273 32.1 20.5 Fuji II 205 15.0 14

Table 1.2: Compressive (CS), flexural (FS) and diametral tensile strength (DTS) of NVP modified commercial glass ionomers [14, 67].

30 1.12 Modifications in GIC powder formulations

The first attempt to increase the strength of conventional glass-ionomer cements

by addition of reinforcing fillers were performed by Simmons in 1983; when he added amalgam alloy powder to GIC powder composition [68]. One of the commercially available samples of this kind of modification is Miracle Mix (MM, GC Corporation,

Japan) which is being widely used in dentistry. However, due to metal/carboxylate matrix interface failure the simple addition of amalgam powder did not exhibit promising results. Subsequently, later studies by McLean and Gasser showed the fused and sintered amalgam powders to glass particles (cermet–ionomer cements) exhibited more promising materials due to strong bonding between the metallic and glass particles [6]. Cermet– ionomer cements had increased resistance to abrasion when compared with glass– ionomer cements and their flexural strength were also higher. However, their strength is still not enough to replace amalgam restoration for posterior teeth [69-74].

Different types of metallic additives have been investigated such as silver, titanium and palladium. Lohbauer et al. in their studies reported that by adding the reactive glass fiber with following composition to the powder of GIC, the fracture toughness of the set glass-ionomer cements increases [69].

Yli-Urpo et al. used bioactive glass particles in the composition of glass- ionomers. They reported that this modification lead to decreased compressive strength,

31 hardness and modulus of elasticity. As a result, clinical usage of bioactive glass particles

ought to be restricted to applications where their bioactivity can be beneficial, such as

root surface fillings and liners [70, 71].

Since hydroxyapatite (HA) has excellent biological behavior, composition and

crystal structure similar to apatite in human dental structure and skeletal system, many

studies have tried to evaluate the effect of addition of the HA powders to the glass

ionomer- cements.

Lucas et al. in their studies added spherical HA particles to the powder of glass

ionomer cements. They reported that, the incorporation of HA particles into the powder

of glass-ionomer cements increased the mechanical properties of the set cement [73].

However in the studies by Gu et al. they indicated that the substitution of GIC glass with

crystalline HA did not affect compressive strength significantly [74].

1.13 Summary and Future Aspects

Compared to other dental materials, glass-ionomer cements have undergone

significant improvements. In particular, glass-ionomer cements have become a topic of interest due to their unique properties. Clinical experiences have confirmed the practical advantages and disadvantages of the glass-ionomer systems. In the case of conventional glass-ionomers, recent studies and trials have resulted in improved formulations,

32 enhanced mechanical properties and reduction of water sensitivity. These improvements have led to more controlled clinical usage techniques. Different studies have shown that there are few ways of reinforcing conventional glass-ionomer cements and gain mechanical properties which are considerably more than the values of non-reinforced ones.

It can be hypothesized that by addition of reinforcing fillers which contain metallic network forming cations or other biocompatible transitional metallic ions, the possibility for formation of more polysalt bridges can be increased, providing a higher level of crosslinkage.

New monomer/polymer combinations should be developed which can improve the strength of the organic matrix within the glass-ionomer cements. Further explorations of other polyelectrolyte systems and reinforcing materials are possible and development of copolymers and additives definitely result in even more effective materials of this type.

33

CHAPTER 2

SIGNIFICANCE AND HYPOTHESES

2.1 Significance

This study was designed to investigate the effects of a novel polymeric

composition on the surface and mechanical properties of conventional glass-ionomer

cements. Novel N-vinylpyrrolidone (NVP) and N-vinylcaprolactam (NVC) containing terpolymers were synthesized, characterized and incorporated into formulations of conventional glass-ionomer cements. The ultimate objective of this research area is to provide potential means for enhancing the mechanical and surface properties of glass ionomer cements in order to make them the material of choice for posterior tooth restorations and as a bone cement for stress bearing areas.

2.2 Hypotheses

The first hypothesis for the current study is that the incorporation of N-vinylpyrrolidone

(NVP) and N-vinylcaprolactam (NVC) co-monomers into the composition of a

34 conventional glass ionomer cement will have major impact on the surface properties (e.g.

contact angle and work of adhesion) of these restorative materials. The second hypothesis

is that these novel polymeric compositions will enhance the mechanical properties (e.g.

fracture toughness, hardness and flexural strength) of glass ionomers. It is expected that

NVC co-monomers would act as a spacer between the itaconic and acrylic acid

monomers, and the degree of steric hindrance would decrease. In addition, the probability of ionic bond formation and subsequent poly-salt bridge formation in the final set glass- ionomer cement will be increased significantly which will affect the surface and mechanical properties. These new polymeric formulations might be able to significantly affect the clinical performance of glass ionomer dental cements.

35

CHAPTER 3

N-VINYLPYRROLIDONE (NVP)-CONTAINING GLASS-IONOMER CEMENTS

3.1 Introduction

One of the most important characteristics of GICs is their adhesion to tooth

structure and their relatively good surface wetting properties as compared to the

hydrophobic nature of composite resins [1-4]. The exact mechanism of adhesion has not

been established for glass-ionomer dental cements. However, studies have shown that

adhesion occurs between glass-ionomer cements and collagen in which pendant COO- groups of collagen can be cross-linked to the GIC matrix by Ca2+ from the glass [9, 10].

It has been indicated that polyacrylic acid (PAA) chains are absorbed on to collagen

through hydrogen bonding and no primary bond is necessary for the formation of polysalt

bridges. Polysalt bridge formation is the mechanism of adhesion of polyelectrolyte

cements to hydroxyapatite, which is also true for the hydroxyapatite component of

dentine [44, 46]. Wilson et al. reported that during adsorption, polyacrylate ions entered

36 the surface of hydroxyapatite by displacing and replacing surface phosphate groups [77].

This is the mechanism of adhesion of polyelectrolyte cement to hydroxyapatite and to the hydroxyapatite component of dentine. Carboxylic acid groups (COO-) react with the

hydroxyapatite of enamel and form ionic bonds, calcium and phosphate are displaced

from the enamel. The result is the formation of an intermediate layer between cement and

the surface of enamel, which is rich in calcium and phosphate [77]. Negm et al. in their

studies reported that the composition of this layer is different from the composition of

enamel, dentine, and glass-ionomer cement [45]. It has been confirmed by IR studies that

strong ionic bonds form between COO- groups of the poly acrylic acid and calcium ions

of the hydroxyapatite in the enamel and dentine structures. Further investigations showed

that the initial adhesion between the glass-ionomer cement and tooth is due to hydrogen

bondings by free carboxyl groups which are presented in the fresh paste. As the cement

matures and becomes harder the hydrogen bonds are progressively replaced by ionic

bonds [42, 78].

NVP (N-vinylpyrrolidone) has the ability to be polymerized with free-radical

initiators, affording a non-ionic, water soluble, synthetic polymer. NVP polymers have

wide range of applications in medicine because of the hydrophilic and non-toxic nature of

the NVP molecule. NVP polymers are easily soluble in water due to some degree of

compensation between the strong hydrogen bonding capability, specifically hydrogen

bonding between the cyclic amide group and water protons, and hydrophobic interactions 37 that exist between water and the NVP polymer backbone and cyclic methylene groups. In addition, NVP-containing polymers have the ability to absorb water [14, 65], where the absorption center is the amide group. In contrast to water-soluble poly(acrylic acid) or poly(methacrylic acid), NVP polymer does not precipitate from aqueous solution, without salts, even upon heating to 100°C [65]. According to previously mentioned properties of

NVP, Culbertson (2001) has reported that NVP is a good comonomer to incorporate in polyelectrolytes glass-ionomer formulations [14].

The ability of NVP-containing polymers to enhance the properties of glass ionomer cements, is probably due to their ability to form hydrogen bonds leading to enhanced surface properties, adhesion and bond strength to dentin. Various reports by

Culbertson et al., 2001, Yamazaki et al., 2005, Xie et al., 1998 [14, 67, 79] and more recently by Moshaverinia et al., 2008 [80] have suggested that NVP modified polyacids can significantly enhance the strength of glass-ionomer dental cements.

It is expected that the presence of NVP molecules in the GIC polyacid composition, has the ability to enhance the surface properties (wetability), surface free energy and bond strength of the resulting material. Hence, the main aim of this study has been to synthesize a NVP modified copolymer of acrylic acid and to assess the effect of this modification on the surface properties (contact angle and work of adhesion) and bond strength to dentin compared to conventional glass-ionomer cements.

38 3.2 Materials and methods

3.2.1 Materials

The glass powders, all liquids and dentin conditioner (GC) which were used in the

experiments were of commercial grade obtained from Fuji II (GC International, Tokyo,

Japan). All the other chemicals in this study were analytical grade and applied as

received from Sigma Adrich Chemical Co. Acrylic acid (AA), itaconic acid (IA), N-

vinylpyrrolidone (NVP), ammonium persulfate, methanol (CH3OH) and anhydrous ethyl

acetate (CH3COOC2H5) were used for polymer synthesis. Commercially available dentin conditioner (Fuji Cavity Conditioner, GC) was used as received from GC International,

Tokyo, Japan.

3.2.2 Methods

3.2.2.1 Synthesis of NVP-containing polymers

Experimental procedure employed in this study is the method reported by Crisp et al., 1980, Yamazaki et al., 2005 and more recently by Moshaverinia et al. [26, 67, 80].

0.4 moles (27.43 mL) of acrylic acid (density of 1.05 gcm-3), 0.05 mole (6.5 g) of itaconic

39 acid and 0.05 moles (5.31 mL) of NVP (density of 1.045 gcm-3) were measured and dissolved in distilled water. Ammonium persulfate (2 % wt) was used as an initiator for the polymerisation reaction. The molar ratio was kept at 8:1:1 for the final product the

AA-IA-NVP terpolymer. The reaction mixture was heated continuously up to 98oC under flowing nitrogen for 12 hours. At the end of this time, the heating was switched off and after cooling for 1 h under nitrogen, the wet polymer was filtered and then freeze-dried.

Freeze-drying was conducted on a Virtis Wizard 2.0 freeze dryer (SP Industries

Co. the Virtis Company, NY, USA) by freeze-drying the polymer at 400 millibars for 22 hours. In order to remove any residual monomers, the dried polymers were completely dissolved in anhydrous methanol and then re-precipitated from anhydrous ethyl acetate

(3×100mL). Yield of the polymer synthesized in water was 65 % overall based on dried mass.

3.2.3 Characterization

1H-NMR analyses of the polymers were conducted using a Burker AV 600 MHz

1 H-NMR using D2O as the solvent (Burker Analytik GmbH, Germany).

FTIR spectra of the synthesized polymers were obtained using a Nicolet 8700

FTIR spectrometer (Thermo Electron Corporation, UK) where the polymer sample films

40 were cast on the KBr crystal to obtain spectra. Spectra were recorded in the mid infrared

region (4000-400 cm-1) at 4 cm-1 resolution and averaging 128 number of scans.

The glass transition of the synthesized terpolymer was measured using differential

scanning calorimetry. The polymeric sample was placed in an aluminum pan in the cell

of the differential scanning calorimetry unit (DSC Q100, TA Instruments, Wilmington,

DE). An empty aluminum pan served as the inert control and nitrogen was used as the

purge gas. DSC was performed from −10 to 150 °C at a heating rate of 10 °C/min. The computer software (TA Universal Analysis 2000) for the apparatus plotted and analyzed

the thermal analysis curves and the values of heat flow were normalized to sample weight

and presented in units of (W/g).

In order to investigate the surface chemistry of the modified set cement, IR

spectra (Perkin Elmer series 2000 FT-IR spectrophotometer with ATR-FTIR attachment)

were obtained for sealed samples having a depth of 1mm and which had been allowed to

cure for a period of one hour. The lower surfaces of the samples were then unsealed and

submerged in distilled water and spectra were subsequently obtained up to one week after

maturation. The IR spectra of Fuji II glass-ionomer cements (control group) were also

obtained for comparison.

Molecular weights of the polymers were estimated by using Zetasizer, nanoseries

analyzer (ZS, Malvern Instruments Ltd. Worcestershire, UK) at 25oC using static light

scattering method. In order to measure the viscosity of the polymers, 1:1 (wt/wt) mixture

41 of polymer and distilled water were first prepared. The viscosities of polymers were measured by application of a programmable reheometer (DV III V3.0, Brookfield

Engineering Laboratories, Inc., Stoughton, MA) at 25oC and spindle rotational speed of

50 RPM.

3.2.4 Formulation and evaluation of surface properties of the modified GIC

3.2.4.1 Specimen preparation

The synthesized polymer was dissolved in distilled water in a ratio of 1:1 (wt/wt).

The glass powder was Fuji II GIC (GC International, Tokyo, Japan) and the powder to

liquid (P/L) ratio of 2.7/1 (w/w) was used as recommended by the manufacturer.

Specimens were hand mixed and fabricated at room temperature following the

manufacturers’ instructions. Five disc shape specimens were molded (d = 10 mm, h = 1.0

mm) using PTFE (poly-tetrafluoroethene) disc shaped moulds for contact angle

measurements. The surface of the samples were polished using median grit silicon

carbide papers (Grade P600, 1500).

In order to measure the bond strength, 35 human extracted or impacted permanent

third molars were stored and surface treated according to the procedures mentioned by

Lucas et al., 2003 [73]. The treated teeth were then mounted in resin holders and both

42 buccal and lingual surfaces of each tooth were trimmed with a low-speed trimmer.

Subsequently, median grit silicon carbide papers (Grade P600, 1500) were used to obtain

smooth dentin surfaces. Both the experimental and also control group (Fuji II) cement

samples were mixed according to manufacturer instructions and applied into a material

holder (with 3.0 mm diameter×3.0 mm height) and samples were fitted by contacting to

the prepared dentin surfaces. The specimen assembly was then stored in 100% relative humidity at 37°C for 1 hour and then in distilled water for 24 hrs, one week and one month.

3.2.4.2 Contact angle measurement

The contact angle of water and α-bromonaphtalene on the surface of the cured glass-ionomer cement samples (disc shaped samples with 10 mm diameter and 1 mm thickness) was measured using static sessile drop method according to the procedure

previously mentioned by Skinner et al. [81]. A KSV CAM-200 optical video contact

angle measuring system was applied in order to measure the water contact angles at 25oC.

The contact diameter of the water drop was determined from a video image of the top

view of the sessile drop. Images were captured during 2 minute with 10 s intervals after a

drop of distilled water was dispensed from the micro-syringe, using a micro video system

(LeicaWild M 32 Microscopy, Wild, Heerbrugg, Switzerland), at a magnification of 10×.

43 Ten consecutive drops of distilled water were deposited with a micro-syringe (Eppendorf,

Hamburg, Germany) on each glass-ionomer sample at 25oC. The average of 15

measurements was taken for both experimental and control groups. Regarding the cosine

of contact angles (θ) of the Zisman series of liquids used in this study and liquid-vapor

A interfacial tension (γ LV) the total work of adhesion (W ) was calculated for each

specimen according to the following equation [82, 83]:

A W = γ LV (1 + cos θ)

In order to investigate the effect of pretreatment of dentin with NVP modified polymer on the wettability of the dentin, human third molars were used according to the procedures mentioned by Rosales-Leal et al., [84]. Radicular portion of the teeth was mounted in self-cured acrylic resin moulds and the occlusal parts of the teeth were cut in order to expose dentin layer. Subsequently the exposed dentin surfaces were polished with 500 grit SiC paper. The same procedure as mentioned above was performed in order to obtain the contact angle of distilled water on dentin surfaces, which were pretreated with Fuji dentin conditioner (which is a mild polyacrylic acid (10% w) solution) as the control group and with synthesized NVP-containing terpolymer.

44 3.2.4.3 Bond strength measurements

After time intervals of 1 hour, 24 hrs, 1 week and 1 month of storage in distilled water, a shear load was applied to the glass-ionomer/dentin interface using standard mechanical testing machine with a knife-edged rod at a crosshead speed of 0.5 mm/min

(all the mechanical testing machines were calibrated prior to start the measurements). The shear force required to separate the cylinder from the dentine was recorded in newtons

(N), and divided by the contact surface area, to determine the shear bond strength value

in MPa. The debonded surfaces of the specimens were air dried and mode of failure was

determined using a SEM (JEOL JSM 6300F High Resolution SEM). The failure mode

was classified according to one of following types: adhesive, cohesive in the cement,

cohesive in dentin or mixed mode of failure. Each specimen was gold coated prior to

analysis. SEM was also applied in order to observe the surface morphology of the

experimental samples and control group.

3.2.5 Statistical Analysis

The data obtained from contact angle measurements, work of adhesion and bond

strength values of specimens were subjected to one- way analysis of variance (ANOVA)

with α = 0.05.

45 3.3 Results

3.3.1 Characterization of synthesized terpolymer

Yield of the polymerization reaction is tabulated in table 3.1 alongside the

molecular weight and viscosity of the final polymeric product.

Copolymers Yield Molecular Weight Viscosity (cP),

% (KDa) 25oC

Poly(AA-IA-NVP) 65 404 1965 ± 17.3

Table 3.1: Yield, viscosity and molecular weight of synthesized NVP-containing

terpolymer.

1 H-NMR spectra of the synthesized terpolymer revealed signals at δ = 1.65 (CH2 from acrylic acid and NVP, respectively), δ = 4.3 (CH from NVP backbone), δ = 3.3

46 (CH2 on NVP ring), δ = 2.3 (CH from acrylic acid) and δ = 3.1 (CH2 of itaconic acid)

which was in good correlation with previously mentioned studies [67, 85].

FTIR spectra (Figure 3.1) of the synthesized terpolymer showed peaks at 1730,

1715, 1650, 1490, 1176 and 1291 cm-1 which are associated with carbonyl group, amide,

-1 CH2 bending and C-N stretch, respectively. The peaks at 1730 and 1715 cm belong to

the carbonyl (C=O) group of carboxylic acid and amide structures. The peaks at 1650,

1490, 1291 and 1176 cm-1 belong to the carbonyl group (amide I) of NVP, C-H stretch

[or (-CH2-) C-H twist], C-N stretch (amide) and CH2 rocking, respectively (see Fig. 1,

below). The FTIR spectrum of the terpolymer shows no peak at 1620 cm-1, associated

with unreacted monomer, indicating that the polymerization reaction has successfully gone to completion. The product also showed a broad peak ranging from 3500 to 2400

-1 cm which is related to C-H stretching from CH3 groups and hydroxyl groups of the

carboxylic acid group [67, 80, 85].

47

Figure 3.1: FTIR spectrum of the synthesized NVP (N-vinylpyrrolidone)-containing acrylic acid copolymer.

Figure 3.2 shows the DSC curves and total heat flow at a heating rate of

10 °C/min for the synthesized terpolymer. There is an endothermic peak at approximately

68oC.

48

Figure 3.2: DSC curve of synthesized AA-IA-NVP terpolymer showing total heat flow at

a heating rate of 10°C/min.

ATR-FTIR (figure 3.3) characterization of the NVP modified GIC and Fuji II

GIC surfaces showed slight band shifts toward lower wavenumbers in the region

49 corresponding to the amide portion of the terpolymer, in comparison to those of the

respective Fuji II polyacid, due to increased hydrogen bond formation. There was also a peak attributed to an amide group that is not present in Fuji II set cement spectrum. The

FTIR spectrum of the NVP modified set cements exhibited peaks at 1580 and 1292 cm-1

which were ascribed to amide groups that were not observed in the spectrum of the Fuji II

GIC.

Figure 3.3: ATR-FTIR spectra of the NVP modified set GIC and Fuji II set glass-ionomer cement after one hour of setting (acid-glass neutralization).

50 3.3.2 Contact angle measurement results

The contact angles of distilled water and α-bromonaphtalene on the surface of

Fuji II GIC and NVP modified GIC are tabulated in table 3.2. In addition, the contact

angle results of solvent drops on the pretreated dentin surfaces with NVP modified terpolymer are also exhibited in table 3.2 and compared with contact angle values of dentin surface pretreated with Fuji conditioner solution. The mean contact angles and work of adhesion values are exhibited in table 3.2. Results showed that incorporated N- vinylpyrroplidone segments in acrylic acid copolymer have the ability to significantly decrease the water contact angle on the surface of the glass-ionomer cements. NVP containing terpolymer also increased the wettability (decreased water contact angle on

the surface of dentin) of the dentin surface in comparison to Fuji dentin conditioner.

51 Sample θ Water θ α-bromonaphthalene γD γP WA

NVP modified GIC 47 (6) 18 (7) 19.8 39.5 59.4 (11.3)

Fuji II GIC 60 (10) 35 (8.8) 13.4 36.8 50.4 (12.5)

Dentin pretreated with

NVP-containing polyacid 21(4) 12 (2.6) 30.5 a 43.6 b 74.2 (18.1) c

Dentin pretreated with

Fuji conditioner 30 (5.5) 18 (1.9) 27.5 a 42.4 b 69.9 (16.4) c

Table 3.2: Contact angles and work of adhesion (both polar and nonpolar components) values of NVP modified glass-ionomer cement, Fuji II glass-ionomer and dentin surfaces treated with Fuji conditioner and NVP-containing polyacid (numbers in parentheses are standard deviations, numbers with same superscripts are not statistically different).

52 3.3.3 Bond strength properties

The bond strength values of the NVP modified GIC and the control group, after 1 hr, 24 hrs, one week and one month of storage in distilled water, are shown in figure 3.4

(below).

Figure 3.4: Bond strength to dentin (MPa) comparison between the NVP modified and

Fuji II commercially available GIC (Results with the same superscript letters are not significantly different (p > 0.05)). 53 The bond strengths of the experimental glass-ionomer samples (7.7 ± 2.0 MPa,

after one week and 7.0 ± 1.2 after one month) after one week to one month of storage in

distilled water were higher than those of the control group (7.3 ± 1.5 MPa for GIC after one week and 6.3 ± 1.2 after one month). There was suggestive but inconclusive evidence of increase in the bonding properties for NVP-containing samples (p=0.09). All of the samples showed cohesive mode of failure in the material. Representative SEM micrographs of the dentin and glass debonded surfaces are shown in figure 3.5 (a and b).

All of the samples showed cohesive mode of failure in the material. Scanning electron micrographs of surface of Fuji II GIC and NVP modified GIC are shown in figure 3.6 (a and b).

54

Figure 3.5: SEM images of the dentin side of a fracture surface from cohesively failed NVP-containing GIC (a) and cohesively failed Fuji II (b) GIC bulk (B) and dentin structure (D) (× 1,500).

55 a)

b)

Figure 3.6: SEM photomicrograph of surface of Fuji II GIC sample (a) and NVP containing GIC sample (b) (× 1,500). 56 3.4 Discussion

The phenomenon in which physical and/or chemical interactions, frequently with

the aid of a surface treatment, hold two surfaces together is described as adhesion [29].

Adhesion means a first state of contact between adherent and adhesive mediated by

physical and chemical forces [86, 87]. The degree of spreading of a liquid on a surface is

a measure of the wettability of the surface by the liquid. This value can be quantified by

contact angle measurements. Contact angle measurements can be employed to monitor

the properties of solid surfaces such as the degree of wetting, the polar and dispersive

surface energies and the critical surface tension [88]. In order to obtain high wettability it

is necessary that the surface energy of the substrate be higher than the surface tension of

the adhesive. If the adhesive has high wettability (low contact angle), there will be a close

contact between the adherent and the substrate and adhesive efficiency will be enhanced

[84-88].

The bonding strength results were in good correlation with the results reported previously by Wilson and McLean, Arora et al. (1998) and Moshaverinia et al. (2008) [1,

59, 89]. Wilson and McLean reported that, the bonding strength of Fuji II GIC to tooth structure is about 5-6 MPa. The bond strength results showed that, there was no significant difference between the bonding strength of NVP modified GIC (6.7 ± 1.3

MPa) and control group (6.4 ± 1.8 MPa) after one hour and 24 hrs of storage in distilled

57 water. Whereas after one month of storage in distilled water considerable difference

between the bond strength values of NVP modified GIC samples (7.0 ± 1.3 MPa) and control group (6.3 ± 1.5 MPa) were observed. It is postulated that this phenomenon is due to more maturation of NVP-containing GIC and presence of more formation of H-bonds on the contact layer between GIC and dentin structure in comparison to Fuji II GIC. NVP molecules in the polymeric structure of glass-ionomer cement composition can form strong hydrogen bonding due to hydrophilic interactions between cyclic amide group and water protons, and hydrophobic interactions that exist between water and the NVP polymer backbone and cyclic methylene groups. Because of the strong hydrogen bond forming capability of NVP it is able to form hydrophilic domains within the matrix of the set cement which not only increases the mechanical properties but also enhance the surface wetting properties of the resulted NVP modified glass-ionomer [14].

The cyclic amide chemical structure of NVP leads the NVP polymers to exhibit strong hydrophilic domains, which have the ability to support localized hydrophilic spheres that increase the bipolar-bipolar forces between the matrix of the glass-ionomer cement and dentin layer. These hydrophilic domains probably are the cause of decreased water contact angle on the surface of NVP modified GIC in comparison to Fuji II.

Consequently, water contact angle results demonstrated that the presence of NVP molecules on the surface of the glass-ionomer samples increased the wettability of the modified cement. A cohesive fracture for glass-ionomer-dentin surface has been reported

58 in literature previously [24, 35]. In this study, scanning electron micrographs of fractured surface of dentin of control GIC showed retained GIC bulk and thin cement matrix

(figure 2.5.a and 2.6.a). Also the surface morphology study of the NVP-containing cements exhibited higher degrees of integrity and lower surface cracks in comparison to

Fuji II samples (figure 2.5.b and 2.6.b).

Dentin wetting was significantly affected by pretreatment of its surface by NVP containing terpolymer. The increased wettability is due to presence of more NVP molecules, more water content and higher degrees of hydration which caused decreased contact angles. This kind of physical bonding is weak, but since there are a large number of these types of bonds, they might be responsible for increase in the mechanical properties of the resulting experimental glasses. Moreover, the possibility of formation of

H-bonds is much more because of the presence of carbonyl groups (amide group) in the matrix of GIC and also in the intermediate layer between GIC and dentin. Stronger bonds between the organic and inorganic network of the set cement, lead to higher mechanical strength of final set cement.

The FTIR-ATR spectra of the NVP modified set cements exhibited peaks at 1580 and 1292 cm-1 which were ascribed to amide II and amide III groups that were not observed in the spectrum of the Fuji II GIC. The aforesaid difference between the spectra indicates the different surface chemistry of the NVP modified glass-ionomer cement in comparison to control group due to the presence of N-vinylpyrrolidone molecules in the

59 experimental GIC polymer. The highest surface tension and total work of adhesion were obtained from the combined use of NVP modified GIC and dentin surface which was treated with NVP-containing polyacid in comparison to the combination of the control groups Fuji II GIC and Fuji conditioner. The amide groups of NVP molecules are potential areas for formation of hydrogen bonds and bipolar intra-molecular interaction which can decrease the contact angle and subsequently increase the work of adhesion and bond strength.

There should be some physiochemical interactions between the carbonyl group of

NVP segments in the terpolymer structure and hydrogen atoms of water (H-bondings), which increased the wettability of the NVP modified surfaces. In addition, there should be some interactions between the modified GIC and dentin structure. Studies have shown that the initial adhesion between the glass-ionomer cement and tooth is due to hydrogen bonding by free carboxyl groups which are presented in the fresh paste [14]. In a synthesized N-vinylpyrrolidone-containing terpolymer both amide groups of NVP molecules and carboxylic acid groups of itaconic and acrylic acid take part in hydrogen bonding formation. As the cement matures and becomes harder the hydrogen bonds are progressively replaced by ionic bonds [79, 80]. In the case of NVP modified GIC, due to higher probability of formation of H-bonds in the intermediate layer between cement and dentin; more ionic bonds of the carboxylate groups would be replaced by hydrogen bonds

60 of the pendant NVP moieties, which consequently increase the bond strength of NVP

modified cement to dentin structure.

3.5 Conclusion

This study revealed that a N-vinylpyrrolidone-containing terpolymer has the

ability to enhance the surface properties of conventional Fuji II glass-ionomer dental

cements such as contact angle, surface chemistry and adhesion to dentin structure. NVP

containing glass-ionomer cements have shown to be a promising dental restorative

material and dentine conditioner, due to their improved mechanical and surface

properties. In future work, we hope to be able to evaluate the biocompatibility and the

amount of fluoride release of NVP (N-vinylpyrrolidone)-containing glass-ionomer cements.

61 CHAPTER 4

N-VINYLCAPROLACTAM (NVC)-CONTAINING GLASS-IONOMER

CEMENTS

4.1 Introduction

The strength of conventional glass-ionomer cements is influenced both by the type of the used glass powder and chemical composition of polyacid used in order to form the organic matrix [14]. Different studies have shown that very close attachments of carboxylic acid groups to the polymeric backbone of PAA, resulted in a very rigid matrix, which causes steric hindrance. Subsequently not all the carboxylic acid groups will convert to carboxylate groups and less polysalt bridge (Ca+2 di and Al+3 tricarboxylate complexes) formation takes place [14, 62]. In order to overcome this problem recent studies have incorporated different monomers such as NVP and amino acid derivatives as spacers between the carboxylic acid groups which made the polymeric backbone more flexible and allow greater access for acid-base reaction (regarding the greater freedom of

COOH groups to react with Al and Ca ions), which results in improved filler-polymer

62 interaction and more complete polysalt bridge formation [14, 62, 63]. This approach was

found to enhance the mechanical strength of glass-ionomer cements [14, 62, 63].

It was found that NVP (N-vinylpyrrolidone) (figure 4.1) modified polyacids

would result in glass-ionomer dental cements with enhanced mechanical strength [67,

80]. Culbertson (2001) reported that acrylic acid-itaconic acid-co-N-vinylpyrrolidone polymers with the different molar ratio have the ability to increase the mechanical properties of the glass-ionomer cements. In another study Yamazaki et al. showed that

AA-MA-NVP (acrylic acid-itaconic acid- N-vinylpyrrolidone) polymer resulted in modified glass-ionomer cements with higher CS and DTS in comparison to Fuji IX commercial glass-ionomer cement [67].

O N

Figure 4.1: Chemical structure of NVP (N-vinylpyrrolidone) molecule.

63 N-Vinylcaprolactam (NVC) (figure 4.2) is a chemical analogue of N-

vinylpyrrolidone (NVP). Applications of NVC in the area of biomedical materials, in

stabilization of proteases and in controlled drug delivery and drug release have been

published previously [90, 91, 92].

N O

Figure 4.2: Chemical structure of NVC (N-vinylcaprolactam) molecule.

N-vinylcaprolactam (NVC) has a ring that consists of seven carbon atoms (one carbon atom more than NVP ring), thus it has the ability to be used in the GIC polyacid structure. It is well known, that the ring of the cycloheptane has the different forms of the conformations as "chair", “bath” and "twist bath" [90-93]. Therefore, it cannot be of the flat conformation and as a result the degree of the steric hindrance in the GIC polyacid

64 structure will be decreased and subsequently an increase in mechanical properties of

NVC-containing GIC will be observed. The polymerisation of N-vinylaprolactam is not

as widely studied as N-vinylpyrrolidone. The NVC can be polymerized by radical

mechanism in or without solvent [93, 94]. NVC polymers have attracted much attention

due to their thermosensitivity and biocompatibility. Due to the fact that hydrolysis of the

amide group of NVC will not produce small amide compounds, NVC is suitable for

biomedical applications [94].

Xie et al. in their studies reported that glass-ionomer samples containing poly

(acrylic acid-co-itaconic acid-co-N-vinylpyrrolidone) with 7:1:3 molar ratios showed

85% higher flexural strength compared to Ketac-Molar (commercially available glass-

ionomer system), with a practical working viscosity [79]. Culbertson and Xie et al. in his

studies synthesized a poly (AA-co-IA-co-NVP) with a 7:3:1 molar ratio of monomers and studied the effects of NVP-containing polymers on the facture toughness and Knoop hardness values of conventional glass-ionomer cements (Fuji II GC) [14, 79]. He reported that the fracture toughness (Klc) for the NVP-containing GIC and Fuji II samples (control

group) were 0.607 and 0.557 Mn/m1/2, respectively. Additionally, the microhardness of

the NVP-containing samples exhibited significantly higher Knoop hardness (KHN)

values (84.3 kg/mm2) in comparison to Fuji II control group (KHN= 77.3 kg/mm2) [14,

95].

65 It was envisaged that NVC molecules interspersed between the copolymers of

itaconic and acrylic acid, would act as a spacer and the degree of steric hindrance would

decrease. In addition, the probability of ionic bond formation and subsequent poly-salt

bridge formation in the final set glass-ionomer cement will be increased significantly.

Since there are more carboxylic acid groups available to make ionic bonds with Al+3 and

Ca+2, there will be more poly-salt bridge formations, and croslinking. Consequently the

mechanical properties of the final set cement will be enhanced. Therefore, the newly

formulated GIC will be the material of choice for posterior teeth restoration and also as

bone graft material in stress bearing areas.

Therefore, the primary objective of this part of the project was to synthesize a

terpolymer of acrylic acid, itaconic acid and N-vinylcaprolactam using free radical

polymerisation technique and to investigate the effect of incorporation of NVC

containing terpolymers into polyacid of GIC, on their mechanical (fracture toughness,

flexural strength and microhardness of GIC) and working properties (setting time and

working time).

In addition, it is expected that the presence of NVC molecules in the GIC

polyacid composition, has the ability to enhance the surface properties (wetability),

surface free energy and bond strength of the resulting material. Hence, the aim of the final section of this study has been to assess the effect of this modification on the surface

66 properties (contact angle and work of adhesion) and bond strength to dentin compared to

conventional glass-ionomer cements.

4.2 Materials and Methods

4.2.1 Materials

The glass powders and polyacid, which were used in the experiments, were

commercially available Fuji IX (GC International, Tokyo, Japan). Acrylic acid (AA),

itaconic acid (IA), N-vinylcaprolactam (NVC), 2, 2 -azobis (isobutyronitrile) (AIBN),

Isopropyl alcohol, methanol and anhydrous ethyl acetate. All the reagents used in this study were of analytical grade and applied as received from Sigma Aldrich Chemical Co.

The details of the chemical used are tabulated in table 4.1.

67 Reactant Molecular Weight Min. Assay % Supplier

Acrylic Acid 72.06 99 Sigma-Aldrich

Itaconic Acid 130.10 99 Sigma-Aldrich

N-Vinylcaprolactam(NVC) 135.14 97 Fluka

2,2 -azobis (isobutyronitrile) 234 99 Sigma-Aldrich

CH3OH 32.04 99.8 Sigma-Aldrich

CH3COOC2H5 88.11 99.8 Sigma-Aldrich

Table 4.1: Reactant details for of AA-IA-NVC polymer synthesis.

4.2.2 Methods

4.2.2.1 Polymer synthesis

The experimental procedure employed in this study is a slight modification of the methods reported by Crisp et al., Yamazaki et al. and Moshaverinia et al., 2008 [26, 67,

80]. Details are as follows: Initially 1% wt of 2, 2 -azobis (isobutyronitrile) (AIBN) as

68 the initiator of the polymerisation reaction was dissolved in 75 mL of distilled water in a

250 ml three neck flask. In the next step, 0.4 moles (27.43 mL) of acrylic acid (density of

1.05 gcm-3), 0.05 moles (6.96 g) of NVC (density of 1.045 gcm-3) and 0.05 moles (6.5 g)

of itaconic acid were measured and dissolved in 37.5 mL of distilled water in a beaker. A

third solution was made up, consisting of 1% wt of 2, 2 -azobis (isobutyronitrile) (AIBN)

dissolved in 22.5 mL of distilled water in a beaker. The first solution was stirred with a

magnetic stirrer (IKA Werke magnetic stirrer/heater) and heated continuously up to 98oC

under constant flow of nitrogen. The second and third solutions were added to the flask after the temperature reached 98oC respectively in a dropwise manner using a glass- dropping funnel at ca. 3ml/min rate. The molar ratio was kept at 8:1:1 for the final product AA-IA-NVC polymer. The solution was stirred vigorously and heated continuously up to 95-100oC under constant flow of nitrogen. Polymerization was

allowed to proceed over night. The polymers were diluted and dissolved in anhydrous

methanol (Sigma-Aldrich) and then precipitated from anhydrous ethyl acetate (Sigma-

Aldrich) in order to remove residual monomers.

4.2.2.2 Characterization of synthesized terpolymer

1H-NMR analyses of the polymers were conducted using a Burker AV 600 MHz

1 H-NMR using D2O as the solvent (Burker Analytik GmbH, Germany). Solution state

69 NMR spectra were recorded in D2O solutions using a 270 MHz JEOL and Bruker

AVANCE 600 spectrometers.

FTIR spectra of the synthesized polymers were obtained using a Nicolet 8700

FTIR spectrometer (Thermo Electron Corporation, UK) where the polymer sample films

were cast on the KBr crystal disks to obtain the results. The polymers were dissolved in

THF in 1:2 w/w of THF and polymer. Spectra were obtained in the mid infrared region

(4000-400 cm-1) at 4 cm-1 resolution and averaging of 256 scans.

In order to investigate the surface chemistry of the modified set cement, IR

spectra (Perkin Elmer series FT-IR spectrophotometer with ATR-FTIR attachment) were

obtained for sealed samples having a depth of 1mm and which had been allowed to cure

for a period of one hour. The IR spectra of Fuji IX glass-ionomer cements (control group) were also obtained for comparison.

Molecular weight of terpolymer was measured by gel permeation chromatography

(GPC, Shimadzu Class VP version 4.2, SHIMADZU Corporation, Kyoto, Japan), using

two Plgel 5 mm Mixed-D 300_7.5mm columns (Polymer Laboratories Inc., Amherst,

MA). The values were relative to the polystyrene standards.

In order to measure the viscosity of the polymers, 1:1 (wt/wt) mixture of polymer and distilled water were first prepared. The viscosities of polymers were measured by application of a cone #6 and plate viscometer (BEL CAP 2000 Viscometer, Brookfield

Engineering Laboratories, Inc., Stoughton, MA) at 25oC.

70 The glass transition of the synthesized terpolymer was measured using differential

scanning calorimetry. A polymeric sample was placed in an aluminum pan in the cell of

the differential scanning calorimetry unit (DSC Q100, TA Instruments, Wilmington, DE).

An empty aluminum pan served as the inert control material and nitrogen was used as the

purge gas. DSC was performed from −10 to 150 °C at a heating rate of 10 °C/min. The computer software (TA Universal Analysis 2000) for the apparatus plotted and analyzed

the thermal analysis curves and the values of heat flow were normalized to sample weight

and presented in units of (W/g).

4.2.2.3 Formulation and evaluation of glass-ionomer samples

4.2.2.3.1 Specimen preparation

Synthesized terpolymers were dissolved in distilled water in a ratio of 1:1 (wt/wt)

proportion. The glass powder was Fuji IX GIC (GC International, Tokyo, Japan) and the

powder to liquid (P/L) ratio of 3.6/1 was used as recommended by the manufacturer.

Specimens were mixed and fabricated at room temperature following the manufacturers’

instructions. For microhardness testing five samples were made from each material for

each time interval (24 hrs and 1 week). Cylindrical shaped samples with 6 mm diameter

and 3 mm thickness were prepared using a Teflon mold. The molds were filled with the

71 uncured cement mixture and covered with PTFE tape and glass slides, flattened and

gently pressed by hand in order to remove air bubbles (N=8). For flexural strength test,

PTFE rectangular shaped specimens with dimensions of 2 mm width by 2 mm depth by

20 mm length were prepared (N= 7). The specimens were removed from the moulds after

20 min and conditioned in distilled water at 37oC for 24 hours and 1 week. For fracture

toughness study, mini-compact disc-shaped specimens with a pre-crack introduced by using a razor blade having 8 × 2 mm dimensions were prepared for NVC-containing glass ionomer samples and the control group, using PTFE split-mould. The specimen were removed from the moulds after 20 minutes and conditioned in distilled water at 37oC for

24 hrs up to one week (N=7).

4.2.2.3.2 Mechanical properties measurements

4.2.2.3.2.1 Microhardness measurements

The Vickers hardness of the specimens was determined according to the method reported by Glayds et al. and Xie et al. [96, 97] using a microhardness tester (Model

MVK-E, M 400, Leco, St. Joseph, MI, USA).The Vickers hardness test was performed using a diamond indenter with 100 g load and a dwell time of 10 s. Each sample was

72 indented two times and the mean hardness value was obtained. The mean Vickers

hardness number for the 5 samples was calculated according to the equation below:

⎡2L× sin (θ / 2)⎤ VHN= ⎣ ⎦ d2 Where:

L= load applied in kilograms

θ= angle of 136o

d= mean diagonal (mm)

Since sin (θ/2) = 1.8544; therefore, the abovementioned equation will be simplified as following formula:

1.8544 × L VHN= d2

Surface hardness of Fuji IX samples (commercially available glass-ionomers) were

evaluated and used as controls.

73 4.2.2.3.2.2 Fracture toughness measurements

Plane strain fracture toughness (K1c) was measured using the technique described by Kovarik and Muncy [98] and more recently Yamazaki et al., 2007 [67] which is in accordance with ASTM Standard 399-05 [99]. Fracture toughness testing was conducted under wet condition using a screw-driven universal testing machine (Model 4202; Instron

Corp, Canton, MA) at cross head speed of 0.5 mm/min according to the method reported by Yamazaki et al. Dimensions of each specimen (B, W) were measured using a traveling microscope (×60) (Nikon Measurescope MM-11; Nikon, Tokyo, Japan). Finally, the fracture toughness (K1c) of each sample was calculated using the following equations:

Pc.(aW) f ⁄ K = 1C BW. 0.5

Where:

Pc is the maximum load before the crack advancement [kN], B is the specimen thickness in cm, W is the perpendicular distance from the unnotched edge of the specimen to a plane defined by the centerline of the loading holes (cm) and F(a ⁄W) is a function of a and W (ASTM Standard 399-83).

74 4.2.2.3.2.3 Flexural Strength Measurements

Flexural strength test was performed on a screw driven mechanical testing machine (Model 4206, Instron Corp., Canton, MA) with the cross-head speed of 0.5 mm/min. Flexural strengths were calculated from the following equation:

3Pl FS = 2Bd 2

Where P is the load of the fracture, l is the distance between the two supports, B is breadth of the sample and d is the depth.

4.2.2.3.2.4 Contact angle measurement

The synthesized polymer was dissolved in distilled water in a ratio of 1:1 (wt/wt).

The glass powder was Fuji IX GIC (GC International, Tokyo, Japan) and the powder to liquid (P/L) ratio of 3.6/1 (w/w) was used as recommended by the manufacturer.

Specimens were hand mixed and fabricated at room temperature following the manufacturers’ instructions. Five disc shape specimens were molded (d = 12 mm, h = 1.0 mm) using PTFE (poly-tetrafluoroethene) disc shaped moulds for contact angle measurements. The surface of the samples were polished using median grit silicon 75 carbide papers (Grade P600). The contact angle was measured 20 s after drop placement utilizing a NRL contact angle goniometer at 26 ± 2 °C (Rame-Hart, Inc; Mountain View,

NJ). The contact angle of water and α-bromonaphtalene on the surface of the cured glass ionomer cement samples (disc shaped samples with 12 mm diameter and 1 mm thickness) was measured using static sessile drop method according to the procedure previously mentioned by Skinner et al. [81]. Please see figure below for further illustration

76 γLV Liquid Drop γSL θ γSV GIC Sample

Young’s Equation

γSV= γSL + cosθ γLV

Where

θ: contact angle between GIC sample and drop of liquid

γSV: solid vapor interfacial free energy γSL: solid liquid interfacial free energy γLV: liquid vapor interfacial free energy

Figure 4.3: Young` s equation for calculation of interfacial surface energy.

Ten consecutive drops of distilled water and α-bromonaphtalene were deposited with a micro-syringe on each glass-ionomer sample at 25oC. The average of 10 measurements was taken for experimental and control groups. According to the cosine of 77 contact angles (θ) of the Zisman series of liquids used in this study and liquid-vapor interfacial tension (γLV) the total work of adhesion (WA) was calculated for each specimen according to the following equation [82, 83]:

WA = γLV (1 + cos θ)

4.2.2.5 Statistical Analysis

One way analysis of variance (ANOVA) followed by tukey`s test was used to determine if there is significant difference between the strengths of the experimental cements and control group. For statistical significance a level of α= 0.05 was used.

4.3 Results

4.3.1 Characterization of synthesized terpolymer

The chemical structure of the synthesized terpolymer is shown in the following figure (figure 4.4):

78 COOH

CH2 CH CH2 CCH2 CH x y z COOH CH2COOH N O

x = 8 y = 1 z = 1

Figure 4.4: Chemical structure of the synthesized terpolymer of acrylic acid- itaconic

acid- N-vinylcaprolactam.

In addition, the results of the terpolymer synthesis i.e. molecular weight and viscosity of the final polymer reaction are given in the Table 4.2. The molecular weight of synthesized terpolymer is significantly higher as compared to the Fuji IX polymer.

Furthermore, the synthesized polymer viscosity value is higher than control Fuji IX polymeric liquid (= 780 cP) when compared to the molecular weight and viscosity values of Fuji IX polymer reported by Yamazaki et al. [67]. Results confirmed that the molecular weight of the synthesized terpolymer and value of the commercially available one are approximately similar to each other and are almost in the same range.

79 Polymer Viscosity (cP) Mw Mn

Fuji IX 850.50 ± 32.1 3.20 × 10 4 9.25 × 10 3

AA-IA-NVC (8:1:1) 2250 ± 82.2 5.32 × 10 4 2.2 × 10 3

Table 4.2: Viscosity and molecular weight values of synthesized terpolymer in comparison to Fuji IX polymer.

The 1H-NMR spectrum of the terpolymer is shown in the Figure 4.5 which displayed signals at δ = 1.65 (CH2 from acrylic acid and N-Vinyl Caprolactam, respectively), δ = 4.8 amide group of N-Vinyl Caprolactam, δ = 4.6 C-H , δ = 4.3 (C-H from N-Vinyl Caprolactam backbone), δ = 3.3 (CH2 on N-Vinyl Caprolactam ring), δ =

2.3 (CH from acrylic acid) and δ = 3.1 (CH2 of itaconic acid). Above mentioned peaks in the spectrum were in good correlation with previously reported data and confirmed the presence of functional groups in the synthesized acrylic acid-itaconic acid-N vinyl caprolactam terpolymer and desired chemical structure of the final terpolymer [67, 80].

80

Figure 4.5: 1H-NMR spectra of synthesized NVC modified polymer.

The FTIR spectrum of the terpolymer (Figure 4.6) shows no peak at 1620 cm-1 due to completion of the polymerization reaction and disappearance of C=C bonds, and consequently the absence of any unreacted monomer, which indicates that the polymerization reaction has successfully gone to completion. The strong peaks at 1734,

1645 and 1540 cm-1 are associated with carbonyl and amide bonds.

81

Figure 4.6: FTIR spectrum of the synthesized NVC-containing terpolymer.

ATR-FTIR (figure 4.7 a and b) characterization of the NVC modified GIC and

Fuji IX GIC surfaces showed slight band shifts toward lower wavenumbers in the region corresponding to the amide portion of the terpolymer, in comparison to those of the respective Fuji IX polyacid, due to increased hydrogen bond formation. The FTIR spectrum of the NVC modified set cements exhibited peaks at 1580 and 1292 cm-1 which were ascribed to amide groups that were not observed in the spectrum of the Fuji IX GIC.

82 a)

b)

Figure 4.7: ATR-FTIR spectra of the (a) NVC-containing terpolymer vs. Fuji IX polymer

(b) NVC modified set GIC and Fuji IX set glass-ionomer cement after one hour of setting

(acid-glass neutralization).

83 Figure 4.8 shows the DSC curves and total heat flow at a heating rate of

10 °C/min for the synthesized terpolymer. There is an endothermic peak at approximately

71oC.

Figure 4.8: DSC curve of synthesized AA-IA-NVC terpolymer with 8:1:1 molar ratio showing total heat flow at a heating rate of 10 °C/min. 84 4.3.2 Evaluation of cements

Results showed that NVC modified glass-ionomer cements exhibited significantly higher fracture toughness (24 hrs: 0.58 ± 0.09 MPa*m1/2 and 1 wk: 0.67 ± 0.2 MPa*m1/2) in comparison to commercially available Fuji IX GIC (24 hrs: 0.43 ± 0.08 MPa*m1/2 and

1 wk: 0.47 ± 0.09 MPa*m1/2). Results of fracture toughness study are summarized in the table 4.3. There was a statistically significant difference between the fracture toughness values of the experimental samples compared to control group (p< 0.05).

GIC Group Fracture Toughness – 24 Fracture Toughness -1wk

hrs (Mpa*m1/2) (MPa*m1/2)

Control Group (Fuji IX) 0.43 ± 0.08 0.47 ± 0.09

NVC-containing GIC 0.58 ± 0.09* 0.67 ± 0.2*†

* p< 0.05 † The large standard deviation (SD) for the experimental sample is due to one data point (outlier) which might have been a defective sample but that it could not be excluded. After the elimination of the outlier the SD would be 0.08.

Table 4.3: Fracture toughness values of experimental and control GIC samples after 24 hrs and one week of storage in distilled water medium at 37oC.

85 The results of Vickers hardness test for the experimental and control samples are shown in table 4.4. The results showed that NVC-containing GIC samples exhibited higher values (VHN: 24 hrs= 54.5 ± 6.0 and 1wk= 60.81 ± 6.2) in comparison to the control (VHN: 24hrs= 52.7 ± 5.40 and 1wk= 57.94 ± 5.54). However, according to the statistical analysis of data there was not a significant increase (p-value > 0.05) in the hardness of GIC samples after 24 hrs and 1 week of storage in distilled water at 37oC.

GIC Group Hardness (VHN) 24 hrs Hardness (VHN) 1wk

NVC-Containing GIC 54.5 ± 6.0 a 60.81 ± 6.2 b

Fuji IX 52.7 ± 5.40 a 57.94 ± 5.54 b

Results with the same superscript letter are not statistically significant.

Table 4.4: Vickers hardness numbers (VHN) of experimental and control GIC samples after 24 hrs and one week of storage in distilled water medium at 37oC.

Results of the flexural strength test have been mentioned in figure 4.9. There is a significant increase in the values of flexural strength in comparison to control group (p-

86 value < 0.05). Results showed that all the glass-ionomer samples matured while immersed in distilled water medium.

NVC Contianing GIC Fuji IX

80 70 60 50 40

FS/MPa 30 20 10 0 FS (24 hrs)/Mpa FS (1 wk)/Mpa GIC Samples

Figure 4.9: Flexural strength values for GIC samples after 24 hrs and one week of maturation in distilled water.

The contact angles of distilled water and α-bromonaphtalene on the surface of

Fuji IX GIC and NVC modified GIC are tabulated in table 4.5. Results showed that incorporated N-vinylcaprolactam segments in acrylic acid copolymer have the ability to

87 significantly decrease the water and α-bromonaphtalene contact angles on the surface of

the glass-ionomer cements.

θ θ 2] Sample Water Bromonaphtalene γD γP WA[erg/cm NVC modified GIC 46a(5.0) 26 (2.7)e 20.14 40.19 60.33a (10.7) Fuji IX GIC 57b (6.1) 31 (3.0)f 14.56 38.45 53.01b (7.3) Dentin surface conditioned with NVC 22c containing polymer (2.4) 12 (1.7)g 30.32 43.46 73.77c(12.0) Dentin surface conditioned with GC 29d(2.0) 17 (1.0)h dentin conditioner 27.85 42.66 70.52d (10.8)

Table 4.5: Contact angles and work of adhesion values of NVC modified glass-ionomer cement, Fuji IX GP glass-ionomer and dentin surfaces treated with GC dentin conditioner and NVC-containing polyacid (numbers in parentheses are standard deviations, numbers with same superscripts are not statistically different).

88 4.4 Discussion

4.4.1 Evaluation of cements

This study investigated the synthesis of a N-vinylcaprolactam-containing novel polymeric material for use in a conventional glass-ionomer cement. The research was carried out to improve the mechanical strength of conventional glass-ionomer cements by producing materials which are stronger and more resistant to wear, than commercially available products. The setting reaction of glass-ionomer cements involves the liberation of metal cations (Al3+ and Ca2+) which react with carboxylic groups COO-- forming Al3+ and Ca2+ polycarboxylates. The main constituent of the polymeric part of most of the current conventional glass-ionomer formulations consists of a copolymer of acrylic acid and itaconic acid or maleic acid. Carboxylic groups in the copolymer of acrylic acid- itaconic acid attached to the backbone chain are more hindered to react with cations due to very close attachment on the polymer backbone [14]. A considerable number of –

COOH groups remain un-reacted in the set cement matrix even after 24 hours of maturation. The strength of the set cement is compromised due to this steric hinderance which reduces Al3+ and Ca2+ salt bridges in the set cement [14]. It has been reported that polymers which have a flexible spacer that links the main polymer backbone chain to the pendant functionality allows greater mobility of the main backbone chain and/or

89 functionality linked through spacer [10, 14, 85]. This mobility allows achieving desired configuration in the final polymer structure. The function of the spacer group is to remove the steric hindrance of the main chain from the functional group of the side chain.

In doing so, the reactivity of the functional groups linked to the spacer group will be changed and get more liberation to get attached and react during the reaction glass particles. Along with this, the specific characteristics of the tethering unit can be employed to bring about changes in hydrophilicity, viscosity and solubility of the polymer and conformation in solution [62-64].

N-vinylcaprolactam is water soluble and capable of producing a biocompatible polymer. Due to the complexation ability and biocompatibility of N-vinylcaprolactam, it has found uses in a variety of applications [100]. In this study, N-vinylcaprolactam was chosen to copolymerize with acrylic acid and itaconic acid. N-vinylcaprolactam is strongly water soluble due to its hydrophilic nature. The cyclic amide group forms hydrogen bonds with water. Due to this strong hydrophilic nature and hydrogen bonding,

N-vinyl caprolactam does not precipitate in aqueous solution. Thus it can be hypothesized that it could provide more stable aqueous solution with acrylic acid and itaconic acid. In this study it was hypothesized that N-vinylcaprolactam molecules could allow tethering of the –COOH groups from main back bone chain and attach at various distance from polymer backbone for more flexibility. This causes a more disordered pattern of the backbone chain with carboxylic groups attached at various distances from

90 rigid chain providing more reactivity to the acid and more availability for Al3+ and Ca2+ tricarboxylate complex (salt-bridge) formation. In previous studies, by Xie at al. N- vinylpyrrolidone was copolymerized with acrylic acid with the intent to modify the polymer structure by inducing more tethering to provide a disordered structure for more mobility and liberation of functional groups and to give more stable aqueous solution of acrylic acid copolymers [66]. Xie et al., 1998, Culbertson and more recently

Moshaverinia et al., 2008 in their studies reported that glass-ionomer samples containing poly (acrylic acid-co-itaconic acid-co-N-vinylpyrrolidone) exhibited higher mechanical properties compared to commercial glass-ionomer cements (such as Fuji II GC and

Ketac-Molar) [14, 66, 67]. Particularly, Culbertson in his studies reported that NVP containing GIC had significantly higher facture toughness and Knoop hardness values in comparison to conventional glass-ionomer cements (Fuji II GC) [14]. Amino acids have been used by Kao et al. and Wu et al. to tether the acrylic acid-itaconic acid copolymer backbone to liberate more acidic groups for salt bridge formation and have shown significant enhancement in conventional glass-ionomer properties [63, 64].

In this study, the mechanical tests results showed that all the glass-ionomer cements became stronger as they matured after 24 hours to one week of storage in distilled water. All the glass-ionomer cements modified with NVC also exhibited significantly higher fracture toughness (experimental= 0.67 ± 0.2 MPa*m1/2 and control=

0.47 ± 0.09 MPa*m1/2) values and increased microhardness (VHN: experimental= 57.34

91 ± 1.54 and control= 60.81 ± 2.2). After 24 hours of maturation, all the NVC-containing cements used in this study exhibited higher flexural strength values [FS= 53.2 ± 9.8 MPa] in comparison to Fuji IX as the control group [FS= 38.3 ± 9.5 MPa]. The statistical analysis results showed that there were significant differences between the mechanical results (FT, FS) of modified glass-ionomer samples in comparison to control group.

Results were in good correlation with previously published literature [96-101]. NVC molecules in the structure of terpolymer exhibit strong hydrophilic domains which can inhibit the separation of the planes of atoms which subsequently affect the response of the material during fracture toughness, flexural strength and microhardness test [95].

Therefore, higher mechanical strength values were obtained for NVC-containing glass ionomers in comparison to the control group. The result of this study correlated well with reported values of Culbertson et al. and Xie et al. Considerable improvements in mechanical properties of experimental cement may indicate increased homogeneity and degrees of polysalt bridge formations of the final set glass cement. Due to presence of amide groups in the structure of the NVC molecules they are capable of hydrophilic domain formation which in turn cause increased inter and intra-molecular forces within cement matrix. This clearly indicates that the incorporation of N-vinylcaprolactam into the backbone of glass-ionomer polymer has improved the strength of cement.

Furthermore, the possibility of formation of H-bonds is increased due to presence of amide groups and more available carboxylic acid groups in the cement matrix.

92 Undoubtedly, stronger bonds between the organic and inorganic network of the cement caused higher mechanical strength of final set cement. The results of more clinically related mechanical testing investigated in this study are comparable to those documented in literature and also our previous published results [14, 79 and 102].

4.4.2 Surface properties of cements

The phenomenon in which physical and/or chemical interactions, frequently with the aid of a surface treatment, hold two surfaces together is described as adhesion [83].

Adhesion means a first state of contact between adherent and adhesive mediated by physical and chemical forces [82, 83]. The degree of spreading of a liquid on a surface is a measure of the wettability of the surface by the liquid. This value can be quantified by contact angle measurements. Contact angle measurements can be employed to monitor the properties of solid surfaces such as the degree of wetting, the polar and dispersive surface energies and the critical surface tension [88]. In order to obtain high wettability it is necessary that the surface energy of the substrate be higher than the surface tension of the adhesive. If the adhesive has high wettability (low contact angle), there will be a close contact between the adherent and the substrate and adhesive efficiency will be enhanced

[88, 103].

93 NVC molecules in the polymeric structure of glass-ionomer cement composition can form strong hydrogen bonding due to hydrophilic interactions between cyclic amide group and water protons, and hydrophobic interactions that exist between water and the

NVC polymer backbone and cyclic methylene groups. Because of the strong hydrogen bond forming capability of NVC it is able to form hydrophilic domains within the matrix of the set cement which not only increases the mechanical properties but also enhance the surface wetting properties of the resulted NVC modified glass-ionomer [14]. The cyclic amide chemical structure of NVC leads the NVC polymers to exhibit strong hydrophilic domains, which have the ability to support localized hydrophilic spheres that increase the bipolar-bipolar forces between the matrix of the glass-ionomer cement and dentin layer.

These hydrophilic domains probably are the cause of decreased water contact angle on the surface of NVC modified GIC in comparison to Fuji IX. Consequently, water contact angle results showed that the presence of NVC molecules on the surface of the glass- ionomer samples increased the wettability of the modified cement.

The FTIR-ATR spectra of the NVC modified set cements exhibited peaks at 1580 and 1292 cm-1 which were ascribed to amide groups that were not observed in the spectrum of the Fuji IX GIC. The aforesaid difference between the spectra indicates the different surface chemistry of the NVC modified glass-ionomer cement in comparison to control group due to the presence of N-vinylpyrrolidone molecules in the experimental

GIC polymer. The highest surface tension and total work of adhesion were obtained

94 from the combined use of NVC modified GIC and dentin surface which was treated with

NVC-containing polyacid in comparison to the combination of the control groups Fuji IX

GIC and Fuji conditioner. The amide groups of NVC molecules are potential areas for formation of hydrogen bonds and bipolar intra-molecular interaction which can decrease the contact angle and subsequently increase the work of adhesion and bond strength.

There should be some physiochemical interactions between the carbonyl group of

NVC segments in the terpolymer structure and hydrogen atoms of water (H-bondings), which increased the wettability of the NVC modified surfaces. In addition, there should be some interactions between the modified GIC and dentin structure. Studies have shown that the initial adhesion between the glass-ionomer cement and tooth is due to hydrogen bonding by free carboxyl groups which are presented in the fresh paste (Culbertson,

2001). In synthesized N-vinylcaprolactam-containing terpolymer both amide groups of

NVC molecules and carboxylic acid groups of itaconic and acrylic acid take part in hydrogen bonding formation. As the cement matures and becomes harder the hydrogen bonds are progressively replaced by ionic bonds [79, 80, 102]. In the case of NVC modified GIC, due to higher probability of formation of H-bonds in the intermediate layer between cement and dentin; more ionic bonds of the carboxylate groups would be replaced by hydrogen bonds of the pendant NVC moieties, which consequently increase the bond strength of NVC modified cement to dentin structure.

95 4.5 Conclusion

In this study a terpolymer of Acrylic acid (AA)-co- Itaconic acid (IA)-co-N- vinylcaprolactam (NVC) with 8:1:1 molar ratio was synthesized, characterized and incorporated into formulation of Fuji IX commercial GIC. This study showed that addition of NVC into glass-ionomer cements has the ability to significantly enhance the fracture toughness, flexural strength and micro-hardness of conventional GIC. This type of glass-ionomers are promising restorative dental materials with improved mechanical properties.

In addition, this study revealed that N-vinylcaprolactam-containing terpolymer has the ability to enhance the surface properties of conventional Fuji IX glass-ionomer dental cements such as contact angle, surface chemistry and work of adhesion. In future work, we hope to be able to evaluate the biocompatibility and the amount of fluoride release of NVP (N-vinylpyrrolidon)-containing glass-ionomer cements.

96

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