im a dentistry student, i need to know everythin about polymerization shrinkage plz, i'd be grateful?

Answer 1

Uh! shouldn't this be in your dental school textbooks???

Answer 2

Introduction

Existing types of photo-curable resins and composites, especially those based on acrylates and methacrylates, exhibit polymerization shrinkage during the setting process.1 The polymerization shrinkage strain of resin-composites, particularly contraction in a direction normal to the interface of the material with dental tissues, is of critical importance.2 Polymerization stresses in a bonded structure may cause adhesive or cohesive failure and interfacial gap formation or, if adhesion is maintained, deformation of residual tooth structure.3

Once initiated, the initial polymerization of visible light-curing (VLC) resin-composites proceeds rapidly shrinkage reaction. For most materials, however, the normalized overall decrease in free-volume (V), i.e., shrinkage, is approximately represented by the Kohlraush-Williams-Watts [KWW] stretched-exponential relaxation function shown here.4

ΔV / Vo = 1 – exp-(t/τ) β

Legend:

Where 0<ß≤1, typical values for ß being 0.3-0.6.
ΔV is the change in molar volume and Vo the molar volume of the monomer.
t is the shrinkage at time, the over all time-constant, the time for the shrinkage to attain a fraction (1-e-1) of its final time.
These time constants (τ) are obtained for a thin specimen geometry (LO < 1.5 mm) in which light irradiation is optimal.5
Visible light, of the proper wavelength range and sufficient intensity, is essential for adequate polymerization of light-activated composite resins.6 The effect of exposure durations and distance of curing tip on light intensity was reported.7,8,9 It was concluded duration and intensity of visible light exposure are both important factors in composite resin polymerization. The polymerization contraction strain is time-dependent, and a methodology for measurement and analysis of the kinetics of this phenomenon has been described.7 This study was based on a deflection disc system which was first described by Wilson.10 The method has been further refined since 1991 to permit control of the ambient temperature (between 0-60˚C) and of the light intensity.11

The effects of light intensity, temperature, and composition on the polymerization behavior of Bis-GMA/TEGDM copolymerization has been reported.1 They concluded the maximum rate of polymerization was significantly affected by the intensity of the light, and the temperature of the polymerization affected the conversion at which the maximum rate occurred. However, polymerization shrinkage could contribute to improving mechanical properties such as stiffness or hardness of composites resin. Unfortunately, stiffness and shrinkage could cause several clinical problems, such as gap formation at the restoration-cavity wall interface and/or build up stress, which could cause caspal deflection.12

Although polymerization shrinkage of composite resin has been extensively investigated, few studies have been conducted on the effect of temperature and light intensity on polymerization. Therefore, the purpose of this study was to evaluate the effects of temperatures on the polymerization shrinkage of five light-cured composite materials cured at full light intensity, and one representative material was selected to evaluated the effect of temperature and light intensity on the polymerization shrinkage.

CHECK SITE OUT BELOW FOR MORE OUTLINES.

Answer 3

Check out this file:

http://www.ecmjournal.org/journal/supplements/vol005supp01/pdf/vol005supp01a22.pdf

Answer 4

Try these few links

http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=10906694&dopt=Abstract

http://iadr.confex.com/iadr/2002SanDiego/techprogram/abstract_17888.htm

Answer 5

http://en.wikipedia.org/wiki/Polymerization
http://www.msel.nist.gov/structure/polymers/techactv95/dentpldnmd.html

Answer 6

maybe this will help u. http://search.yahoo.com/search?search=polymerization+shrinkage&ei=UTF-8&fr=ks-ques&ico-yahoo-search-value=http%3A%2F%2Frds.yahoo.com%2F_ylt%3DAl5bh7BjS795couIkj1pL1MezKIX%2FSIG%3D111gjvvgj%2F*-http%3A%2F%2Fsearch.yahoo.com%2Fsearch&ico-wikipedia-search-value=http%3A%2F%2Frds.yahoo.com%2F_ylt%3DAvHZ.9SihPk3In7maqeZNDAezKIX%2FSIG%3D11ia1qo58%2F**http%253a%2F%2Fen.wikipedia.org%2Fwiki%2FSpecial%253aSearch&p=polymerization+shrinkage