Influence of the colloidal crystal-like arrangement of polymethylmethacrylate bonded aerosil particles on the polymerization shrinkage of composite resins

doi:10.1016/j.dental.2005.08.005

Dental Materials

Volume 22, Issue 7, July 2006, Pages 661-670

https://doi.org/10.1016/j.dental.2005.08.005 Get rights and content

Summary

Objective

Determination of the roles on the polymerization shrinkage during light-curing of composite resins of (i) the filler structuring effect of methylmethacrylate polymer adsorbed on fractal silica particles and (ii) of the crystal-like arrangement of the filler within the matrix.

Methods

Firstly, the domain of major efficiency of polymer in the aerosil agglomeration was determined. Secondly, the monomer system was changed in order to implement the spontaneous concentration of the filler within in a crystal-like arrangement. Finally, the rate of the polymerization shrinkage during light-curing of the resin was determined on the sediment recovered after centrifugation and on the colloidal crystal. Micro-modifications to the size of the samples were determined with the aid of the CCD Line Scan Sensor (H.-D. Rudolph GmbH, Germany).

Results

The composite micro-structure and the rate of polymerization shrinkage during light-curing of the resin are correlated with the amount of polymer initially supplied to the system that controls the degree of filler agglomeration. Different types of matrix/filler systems with typical shrinkage characteristics were sampled from the sediment containing amorphous and organized phases. The colloidal crystal-like system was determined to concentrate the filler within the matrix, and to give rise to the smallest shrinkage.

Significance

This study suggests that the silica particles embedded in a micro-structured polymer network (the filler scaffold) slowly develop spontaneously towards a colloidal crystal-like arrangement, and opens up new processes to reduce polymerization shrinkage.

Introduction

Tooth-colored restoration has promoted increasing interest in the development of resin-based restorative materials that are now widely used instead of amalgam [1], [2]. One of the problems with composite restorative materials is related to polymerization shrinkage, which limits durable adhesion to enamel and dentin [3], [4], [5]. Since bonding to enamel is accepted as clinically reliable, bonding to dentin is more difficult to realize [6], [7], [8]. Therefore, increasing interest addresses the reduction of polymerization shrinkage. One way to reduce polymerization shrinkage has been the incorporation of organic and inorganic fillers of various geometries and sizes. One of the problems with this method is the detrimental effect on mechanical properties [9]. This was attributed to the absence of filler/resin interactions that might be attributed to the chain incompatibility of polymer alloys and blends [10]. Additional information on the role of the filler/matrix interface is given by the study of Luo et al. relative to nanoporous silica gels [11], [12]. Silica gels are synthesized by acid hydrolysis of organosilicates in alcohol (sol–gel process) and recovered after drying as porous particles. Luo et al. determined the influence of silica silanation on the wear resistance of the composites obtained by polymerization of the triethyleneglycoldiacrylate monomer. Insofar as the dose, the specific area and the porosity of the filler exerted a favorable or unfavorable role on the wear resistance, the filler silanation procedure was determined to not change this characteristic.

There is some analogy with aerosils that were generated in the furnace by aggregation of nanoparticles [13]. The influence of the nature of the filler/matrix interface was investigated employing aerosils as filler particles [14]. Two surface treatments were elaborated with functionalized or bare silane coupling agents. No definite conclusion was drawn from the wear resistance tests. Actually, resins enclosing fillers treated with functionalized coupling agents were more resistant than resins enclosing fillers without surface treatment. Conversely, composites enclosing fillers treated with bare silane coupling agents did not present an improved wear resistance. This was attributed to the filler agglomeration that resulted from silanation treatment. This process impeded the homogeneous distribution of the filler particles within the matrix and was assumed to limit the composite performance as determined by Ikejima [15].

More sophisticated fillers like silica-fused whiskers were implemented to reinforce the adhesion of the filler to the matrix [16]. The presence of nano-beads on the surface of the whiskers limits their usual entanglements and improves the homogeneous distribution of the filler within the resin.

Finally, these investigations provided the following information. The performances of some composites cannot definitely be attributed to certain morphology or size of the filler or to specific filler/matrix interactions. All these characteristics might interfere in a complex way and finally give rise to a composite presenting reasonable mechanical properties: the best resin reinforcement resulted from the incorporation at the optimal dose of particles of small size that were homogeneously distributed within the resin. Unfortunately, this assertion contravened some conclusions derived from a recent numerical simulation study [17]. Lusti et al. determined that the rigidity of the composite material did not depend on the homogeneous or inhomogeneous distribution of the filler within the resin. Conversely, the incorporation of aggregated particles induced better wear resistance.

In this paper, it is reported that (i) the use of fractal aerosils agglomerated by addition of polymethylmethacrylate (PMMA) polymer as reinforcing material (the filler) and (ii) the choice of a suitable monomer mixture permitting concentration of the filler within the matrix which would serve to strongly reduce the polymerization shrinkage. The two processes are based on the following approaches:

(i)
Conversely to low molecular weight coupling agents that may change the filler/matrix interactions at the nanometer scale, macromolecules adsorbing on the particles are expected to generate multiple bridges between neighbor particles as well as multiple loops protruding towards the monomer at the hundred of nanometers scale. Obviously, as expected from the results of the various studies relative to the destabilization of colloidal dispersions in the presence of polymer, the relative dose of the polymer should exert the major role. The extent of surface covering and the reconformation of the adsorbed polymer actually control the interfacial polymer conformation and finally the aggregate characteristics that were resumed in the relative sediment height as a function of the polymer dose [18]. The present first investigation focused on the rate of polymerization shrinkage of composite resins with different polymer doses. The dose of polymer was modified from 0 (the reference material) to 45 mg/g of filler. The adsorbed polymer was expected to make compatible matrix and filler, to firmly hold together the agglomerated particles while locally exerting the role of inter-particle springs that oppose the shrinkage of the scaffold.
(ii)
A great number of investigations deal with the influence of the filler dose on the resin characteristics and especially on the polymerization shrinkage. Recently, the characteristics of condensable/flowable and packable composites that contain higher filler doses have been determined [19], [20]. The present second investigation focused on the effect of the colloidal crystal-like arrangement of the filler particles on composite shrinkage. The colloidal crystal-like arrangement slowly developed spontaneously within the settled sediment that resulted from the addition of PMMA to the filler suspended in the resin monomers.

Since the arrangement was observed to particularly develop in the presence of a small amount of added PMMA—but not exclusively—the objective of the study was to determine the characteristics of the final composite that combined the advantages resulting from the inter-filler polymer bonding and from the colloidal crystal-like arrangement.

Section snippets

Resin preparation

The polymethylmethacrylate (PMMA) of molecular weight close to 1.5×10⁶ Da was synthesized in the laboratory and extensively purified in order to eliminate residual monomers. All the other chemical products were from Sigma-Aldrich and were used as received. The experiments were carried out at 25 °C. Among the different fractal powders (aerosil Degussa) already employed as fillers in composite resins, the OX 50 sample (50 m²/g) was selected due to the strong interaction being developed between the

Results

Fig. 3 shows the relative height of the sediment H_∞/H₀ as a function of the amount of PMMA initially supplied to 0.5 g aerosil suspended in 15 ml EGDMA monomer. The degree of aggregation is maximal at the polymer dose near 4 mg PMMA/0.5 g aerosil. The shape of the curve agrees with the usual representation of the sediment height as a function of the polymer dose. At low surface coating, the aggregation efficiency is limited by the lack of polymer. At higher surface coverage, the polymer layer

Results

As indicated above, the aggregates and the platelets have been collected separately from the settled phase and two types of resin were realized by extrusion after centrifugation of the systems. The characteristics of the two resins were compared to the characteristics of the resin realized from the settled whole phase after centrifugation. Fig. 8 shows the rates of shrinkage for the different resin samples. For the resins realized with the settled centrifuged:

–
whole system sustained a shrinkage

Conclusion

The aerosil/PMMA/EGDMA system was chosen in order to precisely determine the effect relevant to the adsorbed PMMA polymer in the reduction of polymerization shrinkage. To this end a very reproducible methodology was implemented in order to reproduce the basic resin/filler/initiator/activator system each time a given amount of polymer was supplied to the system. The technique employed to measure the composite shrinkage delivered the maximal precision since the lower beam level was determined

Acknowledgements

J.M. Widmaier is acknowledged for giving the purified PMMA polymer. The authors would like to thank the Degussa Corp. (Frankfurt, Germany) for kindly providing the aerosil samples and the DegréK Soc. (Strasbourg, France) for the free loan of the Aurys visible light-curing unit. The reviewer is acknowledged for his valuable comments.

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Influence of the colloidal crystal-like arrangement of polymethylmethacrylate bonded aerosil particles on the polymerization shrinkage of composite resins

Summary

Objective

Methods

Results

Significance

Introduction

Section snippets

Resin preparation

Results

Results

Conclusion

Acknowledgements

Dent Mater

J Dent

Dent Mater

Prog Energy Combust Sci

Dent Mater

Biomaterials

Adv Colloid Interface Sci

Prog Polym Sci

Dent Mater

Polymer

Powder Technol

J Control Release

Polymer

Internal adaptation and overhang formation of direct posterior resin composite restorations

Clin Oral Invest

The interaction of adhesive systems with human dentin

Am J Dent

Bonding to enamel and dentin/ a brief history and state of the art

Quintessence Inter

The competition between the composite dentin bond strength and the polymerisation shrinkage stress

J Dent Res