Mirko Andreasi Bassi
DDS, Ph. D, Department of Operative Dentistry,University “La Sapienza”, Rome, Italy

Crystallinity of Polymers: Analysis of a Based Oligocarbonate -dimethacrylic Ester Composite Resin.

Abstract
The oligocarbonate-dimethacrylic ester (OCDMA) has been recently introduced as a resin matrix of visible light cured composite in Operative and Prosthetic Dentistry. A previous research (Waknine et al., 1992) demonstrated that OCDMA, when polymerized, is in a semi-crystalline status when compared with a BisGMA polymer, which has to be considered in an amorphous state. This research evaluated the crystalline properties of a composite, containing as main resin components: UDMA, OCDMA and TEGDMA (Conquest Crystal, Jeneric Pentron Inc., Wallingford, CT, USA), after curing using three different techniques; said analysis has been performed with Differential Scanning Calorimeter (DSC) and Polarized Light Microscopy (PLM). DSC analysis showed a continuos exothermic degradation in all the samples, increasing by heating process, but without showing any melting endothermic peaks. The PLM analysis showed an amorphous polymeric matrix and this confirms the DSC findings. These results suggests that Conquest Crystal has an amorphous resin matrix and its polymeric structure is highly reticulated, impeding therefore any polymeric parallel chains alignment typical of crystalline polymers. This fact is supposed to be due to the other acrylic monomers (UDMA and TEGDMA) which create a strongly reticulated polymeric matrix. Key words: Crystalline Polymers, Dental Composites.

Introduction
During the last few years we could see a continuos improvement in the composite resins for dentistry. But said improvement is mainly due to the quality and quantity of the inorganic fillers, while the resinous matrix, based on BisGMA, has remained practically unchanged (Craig , 1981; Peutzfeldt, 1997).

Generally speaking, resinous matrix is normally composed by BisGMA with addition of other methacrylic monomers whose main function is to improving the handling and get an easier incorporation of the fillers; due to their lower density, the most used are triethylenglycol-dimethacrylate (TEGDMA) and urethane-dimethacrylate (UDMA).

We can see on the market other composites which contain UDMA alone, or based on urethane-tetramethacrylate (UTMA) only.

The curing process transforms all these monomers into polymers by opening their carbon-carbon double bonds, forming a macromolecular complex structured in a three-dimensional irreversible net. The three-dimensional network is guaranteed by the double-functionality of BisGMA and of other monomers, that allow the formation of cross bonds between the polymeric chains (Peutzfeldt, 1997).

The mechanical properties of a composite resin are strictly related to the chemio-physical features of its polymeric matrix. In fact, the more regular and compact is the polymeric structure, the greater is its intermolecular attraction force. For this reason, to a higher degree of crystallinity correspond better mechanical properties of the material.

Generally speaking, a partially crystalline polymer, when compared to a completely amorphous polymer, gives: higher tensile strength; better chemical properties (i.e. lower solubility); higher softening temperature (Phillips, 1991; Tashiro K, Tadokoro H, 1989).

BisGMA and the other above mentioned acrylic monomers can form completely amorphous branched polymers without any repetitive structural scheme, typical of the crystalline materials.

Only linear polymers can show a partial crystallization because some of their polymeric chains have a parallel alignment, with a regular spatial order of the macromolecules.

The molecular structure of the polymers is developed in many directions, and that explains why they can not crystallize in the same way as metals and some low molecular weight substances, and even in the last we can see some crystallization , but it is never complete, as we always find some areas with amorphous structure. The crystallized areas in polymeric structure are known as crystallites (Tashiro K, Tadokoro H, 1989), and cannot be find when the polymer has a completely reticulated structure. The crystallinity degree of a polymer is defined as the percentage in weight of material that is in a crystalline state. For the linear polymers, formed by partially regular chains, a degree of crystallinity of about 90-95 % can be reached (Tashiro K, Tadokoro H, 1989).

Waknine et al. (1992) found that OCDMA has a "semi-crystalline" structure when compared with BisGMA, and therefore can adsorb better those mechanical solicitations that may generate fractures of the material.

This in-vitro study is finalized to the structural analysis of the resinous matrix containing OCDMA, UDMA and TEGDMA contained in a lightcurable dental composite. This material (Conquest Crystal, Jeneric/Pentron, Inc., Wallingford, CT, USA) is indicated for direct restorations, according the manufacturer’s instructions. The chemio-physical characteristics of this material are similar to those of another composite resin (Conquest C/B, Jeneric/Pentron, Inc., Wallingford, CT, USA), employed in prosthetic dentistry (Hisiao and Vijayaraghavan, 1992; Waknine, 1991; Waknine et al., 1991).

Materials and Methods
During the present research, the crystalline properties of the composite under examination have been analyzed after initial lightcuring for 40 s with a curing lamp (Visilux 2, 3M Dental Products, St. Paul, MN, USA) in open atmosphere, at 20 ± 2 °C.

We also evaluated whether it is possible to improve the crystallinity of the material, adding to the lightcuring a treatment of tempering, as the manufacturer suggested for Conquest C/B (Baush et al., 1981). This procedure is called by the manufactures “thermocrystallization” because of the peculiar pseudo-crystalline properties that it confers to the material (Waknine et al., 1992), and is carried out in a special vacuum oven (Curing Unit, Jeneric/Pentron, Inc., Wallingford, CT, USA), at a temperature of 115 °C for 15 min.

In order to verify whether the crystallinity degree of the matrix can be modified by a tempering treatment "not dedicated", some lightcured specimens were exposed to a cycle of additional curing (10 min at 170 °C , 8 atm in watery environment) in an hyperbaric oven (BCG1, Effegi, Sarmato, PC, Italy), designed to avoid the extreme brittleness of composite resins, when extraorally cured in dry condition (Goracci et al., 1998).

In order to see whether this resin matrix has crystalline properties, we used two instrumental investigation methods: polarized light microscopy (PLM) and differential scanning calorimeter (DSC).

Preparation of Samples and Conditions for LPM Analysis
Six composite specimens (color C3 Vita; Batch No. 501681), of 0,5 mm³ each, were applied on as many glasses for optical microscopy. Then a cover glass was applied on every specimen; each of them was thinned up to 100 µm, measured by an electronic caliber.

Successively the specimens were lightcured by the Visilux 2, afterwards the cover glasses were carefully removed. After the curing two samples did not undergo other treatment, whereas two were exposed to postcuring by the BCG1. The remaining specimens were further cured by the Curing Unit.

After the preparation, the specimens were preserved in dry conditions at 20 ± 2 °C for 12 h.

The samples were then analyzed using a polarized light microscope (Universal, Zeiss, Oberkochen, Germany) using the nicols (polarizer and analyzer) in cross position; part of the analysis was carried out interposing a l-compensator between the nicol analyzer and the objective. The investigation was carried out at different magnifications.

Preparation of Samples and Conditions for DSC Analysis
The variations of the physical properties of a substance induced by change of temperature are measured by thermal analysis (Barrall and Johnson, 1966).

The DSC is a special type of thermal analysis recording the difference between the sample and a thermally inactive material selected as a reference, while both are simultaneously exposed to the same pre-programmed changes of temperature. The sample of reference used is an empty crucible of alumina.

Sample and reference are provided with individual heating elements, so that the same temperature can be maintained in the furnace, even if in the sample any exo-/endothermic reaction can occur.

By continuous and automatic regulation of the power it is possible to obtain an high sensibility and very short answering times (Barrall and Johnson, 1966).

Three specimens for each curing technique were prepared for DSC analysis. The weight of each specimen was determined by a balance (Mettler AE 200, Mettler Instruments Corp., Princeton, New Jersey, USA) and was of 10 ± 2.3 mg. Before the examination, the nine specimens have been preserved in dry environment at 20 ± 2 °C for 12 h.

According the protocol, every specimen was subjected to an heating cycle from 25 ±5 °C to 195 ± 3 °C at 10 °C/min. The employed calorimeter was a Du Pont 9000 (Du Pont de Nemours and Co., Inc., Instrument Products Division, Wilmington, Delaware, USA).

Results

Polarized Light Microscope
Analysis At the PLM analysis no differences can be seen in the samples in spite of different ways of curing adopted; the light, which crosses the nicol polarizer, is completely absorbed by the nicol analyser and then the field appears dark.

On this dark background it can be seen bright filler crystals this is due to their two indexes of refraction (birefringence), so they appear bright when nicols are in cross position (fig. 1, 2).

Fig. 1: Optical micrograph of a Conquest Crystal specimen: the dark background is constituted by the amorphous resin matrix and glassy filler, while the bright spots represent the crystalline filler. Polarized light microscopy, 80X.

Fig 2: The same image at higher magnification. Polarized light microscopy, 200X.

We got the same findings when adding the l-compensator (fig. 3, 5-7).

Fig. 3: Optical micrograph showing some crystals dispersed into the resin matrix. Polarized light microscopy with the addition of the l-compensator, 500X.

Fig. 4: Optical micrograph of a Conquest Crystal specimen: a ceramic fiber dispersed in the resin matrix. Polarized light microscopy with the addition of the l-compensator, 500X.

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