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PubMed1991

Determining rate constants for irreversible polymerization where the initial step and propagation steps have different rate constants: consideration of polyadenylate polymerase.

A new relationship is derived between the amount of monomer incorporated and the amount of initiated primer in an irreversible polymerization where the first step, initiation, has a rate constant differing from the elongation rate constants. It is valid for template directed and template independent polymerization. This relationship can be used in kinetic simulation. It suggests a simpler curve fitting technique to attain rate constants from a relatively small data set. Our analysis reveals some limitations of the model of irreversible polymerization; these limitations have not been obvious previously. For example, the initiation rate constant is not attainable from simple monomer incorporation data. Reliable rate constants can be obtained with minimal time course studies.

Kinetics↗
PubMed2003

Role of amination of a polymeric adsorbent on phenol adsorption from aqueous solution.

Adsorption of seven phenols, one aromatic carboxylic acid and one sulfonated aromatic acid from aqueous solution, by a hypercrosslinked polymeric adsorbent (CHA-111) and the derivative animated by dimethylamine was compared. The results of different adsorption isotherms indicated that amino group on the polymeric matrix played a significant role on adsorption of almost all the employed compounds. For most employed compounds adsorption capacities increase to different degree and significantly for those with lower value of pK(a). It may be attributed to the enhanced adsorbent-adsorbate interaction for amino group introduced on the polymeric matrix. The empirical Freundlich isotherm equation was employed to interpret the adsorbent-adsorbate interaction. The adsorption enthalpy change indicated the uptake of phenols on MCH-111 to be an enhanced physical adsorption because of the hydrogen-bonding interaction. Adsorption kinetic study of phenols on CHA-111 and MCH-111 was also conducted and amino group on the matrix will reduce the adsorption rate for change of pore size distribution and loss of macroprous volume as well as the hydration effect on the surface partly.

Adsorption↗
PubMed1999

Specific IgG response to monomeric and polymeric diphenylmethane diisocyanate conjugates in subjects with respiratory reactions to isocyanates.

BACKGROUND: Isocyanates are a frequent cause of occupational asthma and can also induce hypersensitivity pneumonitis. OBJECTIVES: It is still unclear whether antibodies to diphenylmethane diisocyanate (MDI), which are elicited in some subjects with these conditions, are specific for this type of isocyanate. Moreover, preparation of conjugates to human serum albumin (HSA) with the polymeric formulation rather than monomeric MDI might result in improved detection of antibodies. METHODS: We addressed these issues by testing the sera of 13 subjects with asthma (n = 12) and hypersensitivity pneumonitis (n = 1) induced by MDI (n = 4 or 5, see below) by comparing them with sera obtained from subjects with occupational asthma caused by toluene diisocyanate (TDI; n = 5) and hexamethylene diisocyanate (HDI; n = 2). Conjugate preparations were compared by using SDS-PAGE, absorbance spectral analysis, and isolectric focusing. Immunologic screening was done by ELISA. RESULTS: Specific IgG antibodies that recognize MDI-HSA conjugates were detected in all but 1 of the MDI-exposed workers and could not be found in TDI-exposed and HDI-exposed workers. The levels of specific IgG antibodies were more elevated when tested against the HSA conjugates formed with polymeric MDI compared with the HSA conjugates formed with monomeric MDI. CONCLUSION: This study shows that specific IgG antibodies to MDI appear to be specific for MDI without cross-reactivity with TDI and HDI and higher by use of polymeric rather than monomeric MDI-HSA test antigens.

Adult↗
PubMed2002

Energy dependent polymerization of resin-based composite.

OBJECTIVE: This study explores the relationship between the extent of polymerization and the radiant energy (dose) applied during the photopolymerization of resin-based composites. METHOD: FTIR was used to measure the 5-min and 24-h conversion of four resin-based composites prepared in a thin film and polymerized under conditions of decreasing intensity and a constant exposure time (30s) using a tungsten halogen curing light. The measured conversion was obtained over a wide range of applied radiant energy. Additionally, samples for two of the materials were polymerized at various intensities and exposure times such that the dose remained constant. This process was performed at four dose levels representing approximately 75% of the conversion range. RESULTS: The curing profiles (percent conversion versus applied radiant energy) depict a gradual decrease in conversion with decreasing energy followed by a rapid descent. Though there are differences in the maximum conversion attained between the materials, when conversion is represented as a fractional conversion relative to the maximum 24-h value, their 5-min and 24-h curing profiles appear quite similar. Additionally, very similar conversion was measured when the films were exposed using equivalent doses providing evidence for a reciprocal relationship between irradiance (power density) and exposure time. For the 24-h measurements, statistical equivalence (Fishers protected LSD at the 0.05 level) was noted for most of the combinations of exposure time and power density within a given dose. Generally, the exceptions occurred with the shortest exposure times. SIGNIFICANCE: A reciprocal relationship between exposure time and power density adds significance to the study of conversion as a function of the total applied dose. This relationship establishes the curing profile as a universal correlation between exposure time and power density.

Acrylic Resins↗
PubMed2004

Second generation LEDs for the polymerization of oral biomaterials.

OBJECTIVES: New blue, so called second generation light emitting diodes (LEDs) are now available with a high optical power output. These LEDs will potentially find widespread application in commercially available light curing units (LCUs). This study, therefore, investigated the curing performance of a prototype LCU containing one high power LED and a conventional halogen LCU (Polofil). METHODS: The performances of the LCUs were evaluated by measuring the Knoop hardness and depth of cure of the composites. Three dental composites were selected (Z100, Admira and Revolcin Flow) in a light (A2) and a dark shade (A3.5 or A4), respectively, and were polymerized for 40 s each. RESULTS: The LED prototype (irradiance=901 mW/cm2) achieved a statistically significantly greater (p<0.05) depth of cure than the halogen LCU (irradiance=860 mW/cm2) for all composites. Generally, there was no statistically significant difference in Knoop hardness on the top and bottom of a 2 mm thick disk for the composites Z100 and Admira if polymerized with the LED prototype or halogen LCU. The composite Revolcin Flow, however, showed in general a statistically significant lower Knoop hardness if polymerized with the LED LCU. SIGNIFICANCE: The present study shows that second generation LEDs have the potential to replace halogen LCUs if the composites are selected carefully. Furthermore, this study confirmed that the depth of cure test does not discriminate between LCU's performance for composites containing co-initiators, but the Knoop hardness test does.

Analysis of Variance↗
PubMed2004

Polymerization shrinkage-strain kinetics of temporary crown and bridge materials.

OBJECTIVE: The purpose of this study was to measure the polymerization shrinkage kinetics of four commercially available polymer-based temporary crown and bridge materials, including the effect of ambient temperature. METHODS: Three dimethacrylate-based materials and one monomethacrylate-based material were investigated. The polymerization shrinkage-strains were measured by using the Bonded-disk method with initial specimen temperature at both 23 and 37 degrees C, with values particularly noted at 5, 10, and 120 min after mixing. Five recordings were taken for each material. The progress of the setting reaction and its temperature-dependence were evaluated by the kinetic curves, and net shrinkage and total shrinkage (inclusive of expansion magnitude) of each material were compared by independent sample t-test and one-way ANOVA. RESULTS: Most shrinkage occurred in the first 10 min after mixing although there was an early expansion especially with the monomethacylate in the first 5 min. At 120 min, the net shrinkage-strain at 23 and 37 degrees C of the materials used in this test ranged from 3.54 to 4.13%. The fastest setting dimethacrylate-based material and the monomethacrylate-based material showed higher shrinkage-strain than other materials. No significant differences of net shrinkage-strain were found between 23 and 37 degrees C, but higher shrinkage rates were measured at 37 degrees C than at 23 degrees C. SIGNIFICANCE: The Bonded-disk method is a suitable method for measuring temperature-dependence of shrinkage-strain of polymer-based temporary materials. The dimethacrylate-based materials are preferable to monomethacrylates for temporary restoration as judged by the magnitude of polymerization shrinkage-strain, the majority of which is apparent within 10 min from the start of mixing and may affect the clinical outcome.

Acrylic Resins↗
PubMed1990

Colorimetric changes in composites resulting from visible-light-initiated polymerization.

We evaluated three shades of nine light-cured composites to determine the colorimetric changes which occur as a result of the photo-polymerization reaction. A photo-electric tristimulus colorimeter was used to measure the color of a 0.5-mm-thick sample of composite on two different backgrounds before and after the polymerization process had been initiated. The results showed that each of the photo-initiated composites tested produced a visually significant change in color as a result of the polymerization reaction, regardless of the shade of the backing. In general, the light-cured composites produced a characteristic chromatic shift toward the blue region of color space, which resulted in a perceived decrease in yellow chroma. Therefore, direct shade selection of a resin composite which is more yellow or more chromatic than the tooth which is being restored is recommended to compensate for this characteristic immediate color shift.

Color↗
PubMed1999

In-vitro pulp chamber temperature rise during composite resin polymerization with various light-curing sources.

OBJECTIVES: The purpose of this in vitro study was to measure the pulp chamber temperature increase induced during composite resin polymerization with various visible light-curing units. METHODS: A Class II cavity was prepared in an extracted molar tooth, leaving a dentin layer 1 mm thick between pulp chamber and proximal cavity wall. A 2 mm composite resin layer was applied to the proximal box and light-cured with the selected curing units: Heliolux II (H; 320 mW/cm2), QHL 75 (Q; 505 mW/cm2), Astralis 5 (A; 515 mW/cm2), Optilux 500 (O; 670 mW/cm2), Elipar Highlight (EH; 730 mW/cm2), ADT 1000 PAC (P; 1196 mW/cm2). Light-curing took place for 40 s (H, A, Q, O, EH), 5 and 10 s (P). Measurement of pulp chamber temperature changes (starting temperature: 37.0 +/- 0.1 degrees C) during polymerization was performed with a K-type thermocouple positioned at the pulp-dentin junction. Mean values were calculated from 10 measurements with each light-curing unit. ANOVA and Dunnett t-test were used for statistical analyses. RESULTS: Maximum temperature changes varied significantly depending on the light-curing unit used: 2.9 +/- 0.3 degrees C (H), 4.7 +/- 0.5 degrees C (A), 5.4 +/- 0.3 degrees C (P, 5 s), 5.6 +/- 0.4 degrees C (Q), 6.1 +/- 0.2 degrees C (EH, 2-step mode: 100 mW/cm2 over 10 s, 730 mW/cm2 over 30 s), 6.9 +/- 0.4 degrees C (EH), 7.3 +/- 0.3 degrees C (O), 7.8 +/- 0.9 degrees C (P, 10 s). SIGNIFICANCE: It is concluded that light-polymerization with curing units characterized by high energy output (A,EH,O,P,Q) causes significantly higher pulp chamber temperature changes as compared to the conventional curing light (H). Therefore, clinicians should be aware of the potential thermal hazard to the pulp which might result from visible-light curing of composite resins.

Body Temperature↗
PubMed1999

Effect of MMA-PMMA resin polymerization initiators on the bond strengths of adhesive primers for noble metal.

OBJECTIVES: This study was conducted to investigate the effect of MMA-PMMA resin polymerization initiators on the bond strengths of two adhesive metal primers by evaluating the shear bond strengths of resins of silver-palladium-copper-gold (Ag-Pd-Cu-Au) alloy. METHODS: Three types of MMA-PMMA resins for which the polymerization initiators were TBB, BPO-amine and CQ-amine, and two adhesive primers, Metal PrimerII and V-Primer, were used. A brass ring placed over the nonprimed or primed casting alloy disk surface was filled with each resin. The half specimens were stored in water at 37 degrees C for 24 h. In addition, another half specimens were then immersed alternately in water baths at 4 degrees C and 60 degrees C for 1 min each for 20,000 thermal cycles before shear mode testing at a crosshead speed of 0.5 mm/min. RESULTS: There were no significant differences (p > 0.05) in bond strength between the three types of resins with or without thermal cycling when Metal PrimerII was used. However, when Ag-Pd-Cu-Au alloy was primed with V-Primer, the bond strength of CQ-amine resin was significantly weaker than that of TBB resin. Metal PrimerII was more effective for enhancing the bond strength and the bond strength was not affected by thermal cycling, in contrast to V-Primer. SIGNIFICANCE: The effectiveness of Metal PrimerII to enhance the bond strength is not influenced by polymerization mode of MMA-PMMA resin, in contrast to V-Primer when the resin is bonded to Ag-Pd-Cu-Au alloy.

Adhesives↗
PubMed1999

Consistency in the amount of linear polymerization shrinkage in syringe-type composites.

OBJECTIVES: The purpose of this study was to investigate whether the composite resin in a syringe showed a consistent shrinkage through its content. Additionally, the amount of linear shrinkage was compared between materials. METHODS: Five brands of syringe-type and one brand of carpule-type composite resins were used in this study. To each brand, two to three syringes were assigned. In the carpule-type composite, 15 carpules were used. The linear polymerization shrinkage was measured using a custom-made linometer. In this linometer, the amount of displacement of an aluminum disk, which was caused by the linear shrinkage of composite resin, was recorded by a computer every second for 90 s. RESULTS: The syringe-type composites showed similar consistencies in the amount of linear shrinkage except one. The linear shrinkage of the carpule-type Tetric Ceram showed more consistency compared with syringe-type composites. The amount of linear polymerization shrinkage varied between materials. SIGNIFICANCE: This investigation demonstrates that the use of carpule-type composites is recommended instead of syringe-types, because of the consistency in its linear shrinkage. The custom-made linometer provides an effective way to study polymerization shrinkage.

Analysis of Variance↗
PubMed2001

Enzymatic isolation and structural characterisation of polymeric suberin of cork from Quercus suber L.

An enzymatic method has been used to isolate, for the first time, polymeric suberin from the bark of Quercus suber L. or cork. This was achieved by solvent extraction (dichloromethane, ethanol and water), followed by a step-by-step enzymatic treatment with cellulase, hemicellulase and pectinase, and a final extraction with dioxane/water. The progress of suberin isolation was monitored by Fourier transform infrared spectroscopy using a photoacoustic cell (FTIR-PAS). The material obtained (polymeric suberin (PS)) was characterised by solid-state and liquid-state nuclear magnetic resonance, FTIR-PAS and vapour pressure osmometry, and compared with the suberin fraction obtained by alkaline depolymerisation (depolymerised suberin (DS)). The results showed that PS is an aliphatic polyester of saturated and unsaturated fatty acids, with an average molecular weight (M(w)) of 2050 g mol(-1). Although this fraction represents only 10% of the whole suberin of cork, its polymeric nature gives valuable information about the native form of the polymer. DS was found to have an average M(w) of 750 g mol(-1) and to comprise a significant amount of acidic and alcoholic short aliphatic chains.

Bacterial Proteins↗
PubMed2001

Conversion and temperature profiles during the photoinitiated polymerization of thick orthopaedic biomaterials.

Polymerization of a tetrafunctional monomer was investigated under a variety of photoinitiation conditions to assess the ability to form thick materials in situ for orthopaedic applications. The major biological concerns include local cell and tissue necrosis due to the polymerization exotherm and low conversions at greater depths due to light attenuation through thick samples. Experimental results indicate that depth of cure and temperature rises are controllable by altering the photoinitiator concentration, initiating light intensity, and type of photoinitiator. For example, no measurable conversion was detected at a 1.0 cm depth when polymerization was initiated with 1.0 wt% DMPA and 100 mW/cm2 ultraviolet light, whereas approximately 40% conversion was obtained when the initiator concentration was lowered to 0.1 wt%. This conversion was further increased to approximately 55% when a photobleaching initiator system was employed. At the highest rate of initiation studied (i.e., 1.0 wt% DMPA irradiated with 100 mW/cm2 ultraviolet light), a maximum temperature of approximately 49 degrees C was reached at the sample surface; however, this temperature dramatically decreased to approximately 33 degrees C when the light intensity was decreased to 25 mW/cm2. Finally, dual initiating systems that synergistically combine the advantages of photoinitiation and thermal initiation were investigated.

Biocompatible Materials↗
PubMed2001

A polymeric system for the intra-oral delivery of an anti-fungal agent.

Oral candidal infections are often persistent and intractable and thus the aim of this study was to develop a polymeric sustained release device to improve the topical treatment of these infections. A self curing system based on poly(ethyl methacrylate) and tetrahydrofurfuryl methacrylate (PEM/THFM) was used with chlorhexidine diacetate (CX) added at levels between 0 and 12% w/w. Water uptake by the device was assessed gravimetrically and CX release measured by UV spectrometry. Anti candidal activity was established by culturing azole sensitive and resistant strains of Candida albicans in the presence of the polymeric delivery device with and without CX. Candidal growth was measured by turbidimetry or surviving colony-forming unit (CFU) formation. There was an initial high release of CX over 24 h followed by a slow diffusion up to 7 days. CX inhibited candidal growth and survival markedly in vitro, with the test samples showing less than 0.5 x 10(-7) CFU/ml compared to controls (3-4 x 10(-7) CFU/ml). These results indicate the potential of a chlorhexidine containing PEM/THFM polymeric system in the treatment of persistent candidal infections.

Administration, Oral↗
PubMed2001

Ring-opening polymerization of lactones in the presence of hydroxyapatite.

The ring-opening polymerization of epsilon-caprolactone, delta-valerolactone (VL) and D,L-lactide, respectively, in the presence of different proportions of hydroxyapatite (HA) was catalyzed by stannous (II)octoate (SnOct2) at 130 degrees C and resulted in composites. The lactones were almost completely converted to the polymers within a reaction time of 70 up to 240 min. The number-average molecular weights Mn as determined by size exclusion chromatography decreased with increasing content of HA. The initiating efficiency of HA as calculated from the difference of the polymerization degrees P. obtained with and without HA turned out to be relatively low with ca. 11 to 0.5% for 1 to 80 wt% HA, respectively. For the polymerization of VL, the initiating efficiency of HA was on the average threefold higher. The quantitative proof of non-extractable polymer on HA by means of thermogravimetric analysis, fourier transform infrared spectroscopy with photoacoustic detection and differential scanning calorimetry confirmed the initiating efficiency of HA as mentioned above. This poly(lactone) can be debound from HA by treatment with aqueous HCl. Hence it is assumed to be ionically bound.

Biopolymers↗
PubMed2002

Material properties of polymerized NDGA-collagen composite fibers: development of biologically based tendon constructs.

Methods for stabilizing collagen-based materials with catechol containing monomers were developed in order to produce fibers with mechanical properties in tension comparable to those of normal tendon. Fibers produced from pepsin solubilized, bovine tendon type I collagen were polymerized with the di-catechol nordihydroguaiaretic acid (NDGA). Polymerization was based on the chemical oxidation of the constituent o-catechols to reactive o-quinone functionalities. NDGA caused a dose dependent increase in the tensile strength and stiffness of the type I collagen fibers. A second treatment with NDGA improved the tensile properties significantly. Comparison of the effects of NDGA with those of biologically relevant mono-catechols indicated that the bi-catechol functionality of NDGA was responsible for generation of the superior tensile properties. Elimination of unreacted intermediates from the treated fibers with ethanol increased the effectiveness of the cross-linking process while simultaneously sterilizing the material. Catalyzing oxidation by saturating the reaction buffer with oxygen increased the effectiveness of polymerization and the resulting tensile properties of the treated fibers. The ultimate tensile strength of the optimized NDGA-treated fibers averaged 90 MPa; the elastic modulus of these fibers averaged 580 MPa. Both values are comparable to native tendon. The material properties of the NDGA cross-linked fibers exceed the properties of collagen fibers treated with other cross-linking strategies such as glutaraldehyde and carbodiimide. These results indicate that NDGA cross-linking may provide a viable approach to stabilizing collagenous materials for use in repair of ruptured, lacerated or surgically transected tendons, as well as other biomaterial constructs for surgical repair of musculoskeletal injuries and disease.

Biocompatible Materials↗
PubMed2003

Volumetric contraction and methacrylate conversion in photo-polymerized amorphous calcium phosphate/methacrylate composites.

Because of its relatively high solubility in aqueous media and its rapid transformation to hydroxyapatite, amorphous calcium phosphate (ACP) has been utilized as the filler phase of resin-based bioactive composites that have remineralization potential. The objectives of this study were to determine how various methacrylate resins and various types of ACP fillers affect acrylic vinyl conversion and polymerization shrinkage (PS). Several types of photo-crosslinkable resin systems were prepared and admixed with a mass fraction of 40% of either unhybridized, silica- or zirconia-hybridized ACP. After visible light-activated photo-polymerization ACP composites were assessed by near infrared spectroscopy for degree of vinyl conversion and by mercury dilatometry for PS. It was found for these composites that vinyl conversion was independent of filler type but strongly dependent on the type and composition of the resin phase. PS, on the other hand, showed more complex dependence both on the resin type and composition and, in some cases, on the type of ACP. In order to obtain ACP/methacrylate-based composites with maximal vinyl conversion, resin type and composition are of primary importance. However, in order to minimize volume contraction on polymerization it appears necessary to consider both the resin and filler type of these bioactive composites.

Biocompatible Materials↗
PubMed2003

Polymerization and light-induced heat of dental composites cured with LED and halogen technology.

Most commercial light curing units (LCUs) for dental applications use conventional halogen bulbs. Commercial LCUs using light emitting diodes (LEDs) have recently become established on the market, even though some aspects of their performance have not been fully investigated. Temperature rise of dental composites during the light-induced polymerization is considered to be a potential hazard for the pulp of the tooth. This study, therefore, investigated the temperature rise in three different composites (Z100, Durafill, Solitaire2) in two shades (A2, A4) polymerized for 40s with two LED LCUs (Freelight, custom-made LED LCU prototype) and two halogen LCUs (Trilight, Translux). The Trilight was used in the standard and soft-start mode. The temperature rise within the composites were recorded for 60s with a thermocouple and also observed with a high-resolution infrared (HRIR) camera. The factors LCU (p < 0.0001), composite (p < 0.0001) and shade (p = 0.0014) had statistically significant influences on the temperature rise. All composites cured with the halogen LCUs reached at a depth of 2 mm, a statistically significant higher temperature (p < 0.0001) than those cured with the LED LCUs. Only one composite showed a statistically significant lower temperature rise for the halogen LCUs at the 95% confidence level, when the soft-start mode was used instead of the standard mode. In general, the composites with the lighter shade (A2) reached higher temperatures than the darker shade (A4), if the LED LCUs were used. When the halogen LCUs were used, the situation was reversed, the composites with the darker shade (A4) reaching higher temperatures than the lighter shade (A2). This study showed that a HRIR camera represents a powerful tool for the observation of temperature propagation on small samples. This study also showed that LED LCUs represent a viable alternative to halogen LCUs for the light polymerization of dental composites because of a generally lower temperature increase within the composite.

Composite Resins↗
PubMed2003

Improved reverse thermo-responsive polymeric systems.

Novel reverse thermo-responsive (RTG) polymeric systems displaying superior rheological properties were generated by polymerization of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) segments. Two basic synthetic pathways were followed: (1) The bulk polymerization of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock (Pluronic(RTM) F127) (MW=12,600, 70wt% PEO) with hexamethylene diisocyanate (HDI) and (2) The covalent binding of poly(ethylene glycol) and poly(propylene glycol) chains, using phosgene as the connecting molecule. While in the former, the basic amphiphilic F127 repeating unit is known for its own RTG behavior, the latter polymers consist of segments unable of exhibiting reverse thermal gelation of their own. These new materials achieved viscosities at least 15 times higher than F127, at 37 degrees C. Dynamic light scattering measurements revealed that the microstructures formed by these novel polymers were markedly larger than those generated by PEO-PPO-PEO triblocks. While the size of Pluronic F127 micelles ranged from 15 to 20nm, the higher molecular weight amphiphiles generated much larger nanostructures (20-400nm). Finally, the ability of reverse thermo-sensitive gels to perform as drug delivery systems was exemplified by releasing an anti-restenosis model drug (RG-13577). A 30% P[F127](4) gel delivered the drug over 40 days, whereas a F127 gel having the same concentration released the drug over a 7 days period.

Dose-Response Relationship, Drug↗