Journal of Biomedical Materials Research Part A, Sep 15, 2008
Monomer conversion of adhesives in the hybrid layer is important to the quality and longevity of ... more Monomer conversion of adhesives in the hybrid layer is important to the quality and longevity of the dentin bond. In this study, degree of conversion and relative co-monomer concentrations of both experimental and commercial adhesive resins were determined using Raman spectroscopy. The objectives were to identify stable Raman scattering peaks to use as internal references and to determine the effect of water storage on measured conversion and co-monomer concentrations. The peak at 605 cm 21 did not change throughout polymerization and is associated with monomers in both adhesives. This peak was used as an internal reference for conversion and composition calculations before and after water storage. Conversion of the adhesive resins immediately after photopolymerization was $20% lower than that measured after 24-h water storage. HEMA concentration (relative to bis-GMA) in the adhesive resins immediately after photopolymerization was at least 5 wt % higher than that measured after 24-h water storage. Elution of unreacted HEMA provides a reasonable explanation for the ''supposed'' change in conversion measurements, which is more appropriately described as apparent conversion. This apparent conversion will impact interpretation of physical properties and structure of the polymer, as well as increase the probability of water penetration and its reaction within the hybrid layer.
Strategies for designing effective multimedia educational materials for lessons that are delivere... more Strategies for designing effective multimedia educational materials for lessons that are delivered over the Internet will be illustrated using lessons developed for the sophomore-level chemical engineering course on material and energy balances. The advantages offered by the Internet may be exploited to create a valuable educational experience for the student that cannot be duplicated in the formal classroom. These advantages include: i) convenient access to the course from any location and on any schedule; ii) an added level of communication of the scientific concepts through well-designed audiovisual content (including voice, simulations, animations, pictures, and video); iii) the students' control of the pace of the course; and iv) the ability to easily integrate problem solving with the "lecture" component of each lesson. We have developed an Internet version of CHE 201: Material and Energy Balances that illustrates some of the potential for delivering college courses over the Internet. Features of this course include: i) hour-long lessons that are delivered using voice, text, pictures, simulations and animations; ii) sets of 10 to 20 short answer questions that the students encounter about every 20 minutes and are designed to reinforce the concepts that they just learned; iii) an electronic bulletin board (called the WebTalk discussion system) that everyone can access which includes student questions that are updated and answered daily; iv) weekly homework assignments (with posted solutions), and quizzes as well as regular exams that are administered by an approved proctor; v) a cooperative group project in which teams of students work together over the Internet; and vi) a hypertext glossary that can be accessed by clicking on the word to be defined or by moving to the glossary web page.
To directly test the effectiveness of ethanol-wet bonding (EW) in improving monomer infiltration ... more To directly test the effectiveness of ethanol-wet bonding (EW) in improving monomer infiltration into demineralized dentin through quantitative measurement of bis-GMA and TEG-DMA molar concentrations within hybrid layers, and to comprehensively evaluate the effect of EW and chlorhexidine on durability of resin-dentin bonds compared to conventional water-wet bonding (WW). A three-step etch-and-rinse adhesive (70% bis-GMA/28.75%TEG-DMA) was applied to coronal dentin using a clinically relevant ethanol-wet bonding protocol (EW) or the conventional water-wet bonding (WW) technique. Bis-GMA and TEG-DMA molar concentrations at various positions across the resin/dentin interfaces formed by EW and WW were measured using micro-Raman spectroscopy. The experiment was repeated at the same positions after 7-month storage in phosphate buffer solution containing 0.1% sodium azide. The μTBS and hybrid layer morphology (TEM) of bonding groups with and without chlorhexidine application were compared immediately and after 1-year storage in terms of nanoleakage, collagen fibril diameter, collagen interfibrillar width, and hybrid layer thickness. Specimens bonded with EW showed significantly higher monomer molar concentrations and μTBS throughout the hybrid layer immediately and after storage, providing direct evidence of superior infiltration of hydrophobic monomers in EW compared to WW. Microscopically, EW maintained interfibrillar width and hybrid layer thickness for resin infiltration and retention. The application of chlorhexidine further preserved collagen integrity and limited the degree of nanoleakage in EW after 1-year storage. EW enhances infiltration of hydrophobic monomers into demineralized dentin. The results suggest that a more durable resin-dentin bond may be achieved with combined usage of a clinically relevant EW and chlorhexidine.
Encyclopedia of Polymer Science and Technology, Mar 15, 2004
Free-radical photopolymerizations, which use light energy to initiate chain-producing reactions, ... more Free-radical photopolymerizations, which use light energy to initiate chain-producing reactions, have many advantages over thermal polymerizations, including solvent-free systems, spatial and temporal control of initiation, and high speed processing capabilities. This article provides an overview of the components, kinetics, and applications of free-radical photopolymerization systems. Two general classes of photoinitiator systems (unimolecular and bimolecular) are discussed, as well as systems that have been developed to meet the demands of specific applications. (Meth)acrylates, which are the most widely used monomers in photopolymerization processes, and other commercially important free-radical monomers, are described. The kinetic treatment of free-radical photopolymerizations is outlined, with special emphasis on the photoinitiation step and its impact on the rate of polymerization expression. Kinetic modeling concerns in reaction systems containing monomers with more than one reactive group and in systems utilizing specialized polymerization methods are also considered. Finally, applications for free-radical photopolymers in the automotive, electronic, medical, optical, graphic arts, flooring, and furniture industries are presented. This article provides the scientist or engineer with fundamental considerations of free-radical photopolymerizations, along with references for more advanced topical research. Keywords: photopolymerization; photocuring; free radical; photoinitiators; monomers; kinetics
Journal of Biomedical Materials Research Part A, Feb 22, 2010
The objective was to determine absolute molar concentration of adhesive resin components in the h... more The objective was to determine absolute molar concentration of adhesive resin components in the hybrid layer by establishing methods based on Raman spectroscopy fundamentals. The hybrid layer was treated as a three-component system consisting of collagen and an adhesive resin containing two monomers. Adhesive standard specimens and Raman peak area ratios obtained with a 785 nm excitation wavelength were used to construct separate calibration curves for comonomer relative molar concentration and Bis-GMA absolute molar concentration. Since collagen and water had no measurable peaks in the fingerprint region, a dilution coefficient K j was defined to describe their impact on Raman peak area and to calculate HEMA absolute molar concentration. Methodology was validated using an analogous system containing acetone/ethanol/ water. The absolute molar concentration of Bis-GMA and HEMA decreased 87% and 83%, respectively, from the top quarter to the middle of the hybrid layer. Additionally, less Bis-GMA penetrated the hybrid layer than HEMA, as indicated by the ∼20% decrease in comonomer molar concentration ratio between the adhesive resin layer and the top half of the hybrid layer. Lack of complete monomer infiltration will further challenge dentin-adhesive bond longevity.
The Emergency/Homeless Shelter Design Project was developed for "Engineering Problem Solving I", ... more The Emergency/Homeless Shelter Design Project was developed for "Engineering Problem Solving I", a freshman engineering course, based on an exercise presented in Engineering Your Future: A Project-Based Introduction to Engineering. The purpose of the project is to encourage students to apply their design skills to serve society. Students are challenged to design a cardboard structure to keep people warm in cold weather, which will be warmed with sunlight and body heat. Classroom activities that help the teams in studying the issues of housing design include a guest lecture by the CEO of PEER Consultants, P.C. (an international civil and environmental engineering consultancy), brainstorming sessions to generate ideas for the design, a virtual building assignment using Energy-10 software to evaluate the energy efficiency of a variety of building materials, and lectures on oral and written communications of technical material. Teams of four first-year students write a proposal for the design of a low-cost shelter and then build and test a prototype of the structure using a basic kit provided to them. The designs are demonstrated during the final week of classes in a poster session where students, faculty and staff are invited to view the shelters and technical posters describing the designs and testing results.
Photopolymerization systems based on hybrid monomer 3,4-epoxy-cyclohexylmethyl methacrylate (METH... more Photopolymerization systems based on hybrid monomer 3,4-epoxy-cyclohexylmethyl methacrylate (METHB) were studied to investigate water effects on conversion and polymer coating properties. METHB contains epoxide and methacrylate moieties, which undergo cationic and free-radical photopolymerization, respectively. The conversion of both groups was obtained by Raman spectroscopy in real time and depth. Water concentration and initiator system compositions were varied and shown to affect reaction kinetics and depth profile. With increasing water concentration, the epoxide induction period increased when only cationic initiator was present; however, the induction period disappeared when using the dual-initiator system. In addition, epoxide groups continued reacting after light was shuttered and reached a higher and more homogeneous conversion. Hybrid systems were shown to be less sensitive to water at low or intermediate concentrations. With high water concentrations, hybrid monomer systems manifested increased ring opening during illumination due to chain transfer and decreased physical properties due to loss of cross-linking.
Starch‐g‐poly(methyl methacrylate) (PMMA) was prepared by emulsion photopolymerization without ph... more Starch‐g‐poly(methyl methacrylate) (PMMA) was prepared by emulsion photopolymerization without photoinitiator. Grafting efficiency was determined for two types of starch (high‐amylose and amylopectin) and at several illumination times. Since clogging of the graft copolymer prevented vacuum filtration and Soxhlet extraction was too time‐consuming, a new method to separate the PMMA homopolymer from the graft copolymer was developed. Back‐flush filtration uses an intermittent pressure pulse to clean the filter as the homopolymer is separated from the graft copolymer. Back‐flush filtration was shown to be more efficient by reducing the separation time and solvent use, and the grafting efficiencies obtained with back‐flush filtration compare favorably with those from Soxhlet extraction for the starch‐g‐PMMA copolymer systems studied. More accurate grafting efficiencies could be obtained by applying a nonstick coating to the inside chamber of the back‐flush filtration unit.
C M-constant for chain transfer to the monomer DPPH-2,2-diphenyl-1-picrylhydrazyl EB-electron bea... more C M-constant for chain transfer to the monomer DPPH-2,2-diphenyl-1-picrylhydrazyl EB-electron beam G i-number of ions formed per 100 eV of energy absorbed by the system G(-m)-number of bonds broken per 100 eV of energy absorbed by the system G(M •)-number of initiating radicals formed per 100 eV of energy absorbed by the system G(R •)-number of primary radicals formed per 100 eV of energy absorbed by the system GPC-gel permeation chromatography k p-propagation rate constant k t-termination rate constant M •-initiating radical [M]-monomer concentration MMA-methyl methacrylate R p-rate of propagation R irate of initiation R R-rate of radical formation R •-primary radical X n-number-average degree of polymerization
Journal of Biomedical Materials Research Part A, Feb 2, 2010
Penetration of adhesives into the demineralized dentin surface and their subsequent conversion ar... more Penetration of adhesives into the demineralized dentin surface and their subsequent conversion are critically important to longevity of the adhesive resin-dentin bond. The durability of the resindentin bond is investigated by monitoring the change of adhesive concentration within the hybrid layer of aged specimens using Raman spectroscopy. Absolute molar concentrations of Bis-GMA and HEMA were measured across the hybrid layer of resin-dentin specimens 24 h after photopolymerization and after 24-wk storage in one of three media: artificial saliva (SAL), SAL containing cholesterol esterase to attack resin (EST), and SAL containing bacterial collagenase to attack collagen (COL). No significant difference among these groups for both Bis-GMA and HEMA molar concentrations at 24-h storage was found; however, concentrations decreased from the adhesive resin to the middle of the hybrid layer. Concentrations remained unchanged at any resin-dentin position after aging in SAL. In the hybrid layer, concentrations significantly decreased with aging in COL and tended to decrease in EST. While showing potential enzymatic biodegradative effects of endogenous matrix metalloproteinases and salivary esterases, this methodology may also prove to be a valuable assessment of new chemistries and future approaches to improve resin-dentin bond performance.
Photopolymerization systems based on hybrid monomer 3,4-epoxy-cyclohexylmethyl methacrylate (METH... more Photopolymerization systems based on hybrid monomer 3,4-epoxy-cyclohexylmethyl methacrylate (METHB) were studied to investigate the oxygen inhibition effects on the conversion and polymer properties in films and coatings. METHB contains epoxide and methacrylate functional groups, which undergo cationic and free-radical photopolymerization, respectively. The conversion of both groups was obtained by Raman confocal microscopy as a function of depth. Initiator system compositions were varied and shown to affect the depth profile. The methacrylate group conversion was low at the surface due to oxygen inhibition. When both reactions were present, a cross-linked network formed and reduced oxygen sensitivity. At depths greater than the oxygen-diffusion-affected region, both functional groups' conversions did not show depth dependence. In addition, epoxide groups continued reacting after light was shuttered and reached a higher and more homogeneous conversion. These systems exhibit lower sensitivity to oxygen and offer advantages such as increased cure speed and improved film-forming properties compared to free-radical systems.
poxy resins were cured via UV illumination in atmospheres of varying humidity levels. The chain-t... more poxy resins were cured via UV illumination in atmospheres of varying humidity levels. The chain-transfer reaction between the epoxy moieties and water molecules is described by the activated monomer mechanism. The physical effects of chain-transfer reactions mainly occur at the surface due to limited diffusion during the reaction. Force curves were obtained via atomic-force microscopy and used to determine the surface modulus of cured resins. A decrease in modulus was observed for resins cured in humid atmospheres.
This study examined the adhesive interface created by Filtek Silorane Adhesive System (SIL, 3M ES... more This study examined the adhesive interface created by Filtek Silorane Adhesive System (SIL, 3M ESPE, USA) using micro-Raman spectroscopy. The HL thickness of the adhesive interface created by SIL was comparable with data previously reported on self-etch adhesives, while the adhesive layer was considerably thicker. As previous investigations revealed that high DC is a fundamental pre-requisite for the stability of the resin bond over time, the DC results of the present study support the hypothesis that optimal stability of SIL within the HL can be obtained. However, further studies are needed to investigate the mechanical properties of the hybrid layer created by SIL and its stability over time
Although cationic photopolymerizations are not inhibited by oxygen, polymerization rates and conv... more Although cationic photopolymerizations are not inhibited by oxygen, polymerization rates and conversions of commonly used cycloaliphatic epoxides are typically much lower than industrially prevalent acrylates undergoing free-radical photopolymerization. The addition...
Composites Part A-applied Science and Manufacturing, Jun 1, 2013
The use of thick sections of fiber-reinforced polymers (FRPs) is increasing for numerous industri... more The use of thick sections of fiber-reinforced polymers (FRPs) is increasing for numerous industrial applications such as wind turbine blades. In situ cure monitoring is very important to directly observe the cure process of FRPs during the manufacturing process. In this work, Raman spectroscopy and dielectric analysis (DEA) are investigated for in situ cure monitoring of an epoxy resin. The cure behavior is first characterized using differential scanning calorimetry (DSC) as a baseline comparison, and the best-fit phenomenological reaction model is determined to describe the cure behavior of the epoxy resin as well as the kinetic parameters. The relationship between T g and degree of cure is also established. The degree of cure obtained from Raman spectroscopy and DEA under isothermal conditions is compared to that obtained from DSC. A good agreement is observed among the three methods, supporting the potential of these in situ cure monitoring methods during manufacturing. An implementation plan for in-plant monitoring is also discussed.
Flipping is an appealing method to engage students for meaningful and active learning. However, t... more Flipping is an appealing method to engage students for meaningful and active learning. However, the work needed to generate the content the students will use outside the classroom and to coordinate the activities the students will do inside the classroom seems daunting. This paper provides ideas to reduce the start-up costs in time and energy, essential elements to include in the implementation of the flip, and instructor and student behaviors that may need adjusting for a more effective flipped experience. These helpful hints are based on the author's experience in flipping her sophomore-level chemical engineering required course for the first time last fall.
Electron-beam (EB) and photo-polymerization rely on radiation chemistry; however, fundamental dif... more Electron-beam (EB) and photo-polymerization rely on radiation chemistry; however, fundamental differences arise in initiation and energy deposition. Since photopolymerization has been widely studied, a quantitative comparison would facilitate a better understanding of the kinetic and physical impact of the EB initiation mechanism, which is substantially more difficult to characterize. Here, a protocol was developed that enables investigation of EB and photopolymerized films of equivalent initiation energies. Using this protocol, the impact of the initiation mechanism on energy rate effects (EREs) was characterized for a series of five acrylate monomers. Differences in polymer conversion and physical properties were determined via Raman spectroscopy and dynamic mechanical analysis, respectively. In comparison to the EBpolymerized films of equivalent initiation energy and energy rate, the photopolymerized films had equal or lower conversions. Additionally, monomer chemistry was a key factor for differentiating ERE magnitude across the five-monomer series for both initiation mechanisms. Differences between initiation mechanisms were also demonstrated in T g , FWHM of the tan δ peak, and the maximum tan δ peak height for two of the monomer formulations. Quantifying these differences not only improves knowledge of fundamental kinetics, but also provides a foundation for predictive chemical structure/processing parameter/polymer property relationships.
Journal of Biomedical Materials Research Part A, Sep 15, 2008
Monomer conversion of adhesives in the hybrid layer is important to the quality and longevity of ... more Monomer conversion of adhesives in the hybrid layer is important to the quality and longevity of the dentin bond. In this study, degree of conversion and relative co-monomer concentrations of both experimental and commercial adhesive resins were determined using Raman spectroscopy. The objectives were to identify stable Raman scattering peaks to use as internal references and to determine the effect of water storage on measured conversion and co-monomer concentrations. The peak at 605 cm 21 did not change throughout polymerization and is associated with monomers in both adhesives. This peak was used as an internal reference for conversion and composition calculations before and after water storage. Conversion of the adhesive resins immediately after photopolymerization was $20% lower than that measured after 24-h water storage. HEMA concentration (relative to bis-GMA) in the adhesive resins immediately after photopolymerization was at least 5 wt % higher than that measured after 24-h water storage. Elution of unreacted HEMA provides a reasonable explanation for the ''supposed'' change in conversion measurements, which is more appropriately described as apparent conversion. This apparent conversion will impact interpretation of physical properties and structure of the polymer, as well as increase the probability of water penetration and its reaction within the hybrid layer.
Strategies for designing effective multimedia educational materials for lessons that are delivere... more Strategies for designing effective multimedia educational materials for lessons that are delivered over the Internet will be illustrated using lessons developed for the sophomore-level chemical engineering course on material and energy balances. The advantages offered by the Internet may be exploited to create a valuable educational experience for the student that cannot be duplicated in the formal classroom. These advantages include: i) convenient access to the course from any location and on any schedule; ii) an added level of communication of the scientific concepts through well-designed audiovisual content (including voice, simulations, animations, pictures, and video); iii) the students' control of the pace of the course; and iv) the ability to easily integrate problem solving with the "lecture" component of each lesson. We have developed an Internet version of CHE 201: Material and Energy Balances that illustrates some of the potential for delivering college courses over the Internet. Features of this course include: i) hour-long lessons that are delivered using voice, text, pictures, simulations and animations; ii) sets of 10 to 20 short answer questions that the students encounter about every 20 minutes and are designed to reinforce the concepts that they just learned; iii) an electronic bulletin board (called the WebTalk discussion system) that everyone can access which includes student questions that are updated and answered daily; iv) weekly homework assignments (with posted solutions), and quizzes as well as regular exams that are administered by an approved proctor; v) a cooperative group project in which teams of students work together over the Internet; and vi) a hypertext glossary that can be accessed by clicking on the word to be defined or by moving to the glossary web page.
To directly test the effectiveness of ethanol-wet bonding (EW) in improving monomer infiltration ... more To directly test the effectiveness of ethanol-wet bonding (EW) in improving monomer infiltration into demineralized dentin through quantitative measurement of bis-GMA and TEG-DMA molar concentrations within hybrid layers, and to comprehensively evaluate the effect of EW and chlorhexidine on durability of resin-dentin bonds compared to conventional water-wet bonding (WW). A three-step etch-and-rinse adhesive (70% bis-GMA/28.75%TEG-DMA) was applied to coronal dentin using a clinically relevant ethanol-wet bonding protocol (EW) or the conventional water-wet bonding (WW) technique. Bis-GMA and TEG-DMA molar concentrations at various positions across the resin/dentin interfaces formed by EW and WW were measured using micro-Raman spectroscopy. The experiment was repeated at the same positions after 7-month storage in phosphate buffer solution containing 0.1% sodium azide. The μTBS and hybrid layer morphology (TEM) of bonding groups with and without chlorhexidine application were compared immediately and after 1-year storage in terms of nanoleakage, collagen fibril diameter, collagen interfibrillar width, and hybrid layer thickness. Specimens bonded with EW showed significantly higher monomer molar concentrations and μTBS throughout the hybrid layer immediately and after storage, providing direct evidence of superior infiltration of hydrophobic monomers in EW compared to WW. Microscopically, EW maintained interfibrillar width and hybrid layer thickness for resin infiltration and retention. The application of chlorhexidine further preserved collagen integrity and limited the degree of nanoleakage in EW after 1-year storage. EW enhances infiltration of hydrophobic monomers into demineralized dentin. The results suggest that a more durable resin-dentin bond may be achieved with combined usage of a clinically relevant EW and chlorhexidine.
Encyclopedia of Polymer Science and Technology, Mar 15, 2004
Free-radical photopolymerizations, which use light energy to initiate chain-producing reactions, ... more Free-radical photopolymerizations, which use light energy to initiate chain-producing reactions, have many advantages over thermal polymerizations, including solvent-free systems, spatial and temporal control of initiation, and high speed processing capabilities. This article provides an overview of the components, kinetics, and applications of free-radical photopolymerization systems. Two general classes of photoinitiator systems (unimolecular and bimolecular) are discussed, as well as systems that have been developed to meet the demands of specific applications. (Meth)acrylates, which are the most widely used monomers in photopolymerization processes, and other commercially important free-radical monomers, are described. The kinetic treatment of free-radical photopolymerizations is outlined, with special emphasis on the photoinitiation step and its impact on the rate of polymerization expression. Kinetic modeling concerns in reaction systems containing monomers with more than one reactive group and in systems utilizing specialized polymerization methods are also considered. Finally, applications for free-radical photopolymers in the automotive, electronic, medical, optical, graphic arts, flooring, and furniture industries are presented. This article provides the scientist or engineer with fundamental considerations of free-radical photopolymerizations, along with references for more advanced topical research. Keywords: photopolymerization; photocuring; free radical; photoinitiators; monomers; kinetics
Journal of Biomedical Materials Research Part A, Feb 22, 2010
The objective was to determine absolute molar concentration of adhesive resin components in the h... more The objective was to determine absolute molar concentration of adhesive resin components in the hybrid layer by establishing methods based on Raman spectroscopy fundamentals. The hybrid layer was treated as a three-component system consisting of collagen and an adhesive resin containing two monomers. Adhesive standard specimens and Raman peak area ratios obtained with a 785 nm excitation wavelength were used to construct separate calibration curves for comonomer relative molar concentration and Bis-GMA absolute molar concentration. Since collagen and water had no measurable peaks in the fingerprint region, a dilution coefficient K j was defined to describe their impact on Raman peak area and to calculate HEMA absolute molar concentration. Methodology was validated using an analogous system containing acetone/ethanol/ water. The absolute molar concentration of Bis-GMA and HEMA decreased 87% and 83%, respectively, from the top quarter to the middle of the hybrid layer. Additionally, less Bis-GMA penetrated the hybrid layer than HEMA, as indicated by the ∼20% decrease in comonomer molar concentration ratio between the adhesive resin layer and the top half of the hybrid layer. Lack of complete monomer infiltration will further challenge dentin-adhesive bond longevity.
The Emergency/Homeless Shelter Design Project was developed for "Engineering Problem Solving I", ... more The Emergency/Homeless Shelter Design Project was developed for "Engineering Problem Solving I", a freshman engineering course, based on an exercise presented in Engineering Your Future: A Project-Based Introduction to Engineering. The purpose of the project is to encourage students to apply their design skills to serve society. Students are challenged to design a cardboard structure to keep people warm in cold weather, which will be warmed with sunlight and body heat. Classroom activities that help the teams in studying the issues of housing design include a guest lecture by the CEO of PEER Consultants, P.C. (an international civil and environmental engineering consultancy), brainstorming sessions to generate ideas for the design, a virtual building assignment using Energy-10 software to evaluate the energy efficiency of a variety of building materials, and lectures on oral and written communications of technical material. Teams of four first-year students write a proposal for the design of a low-cost shelter and then build and test a prototype of the structure using a basic kit provided to them. The designs are demonstrated during the final week of classes in a poster session where students, faculty and staff are invited to view the shelters and technical posters describing the designs and testing results.
Photopolymerization systems based on hybrid monomer 3,4-epoxy-cyclohexylmethyl methacrylate (METH... more Photopolymerization systems based on hybrid monomer 3,4-epoxy-cyclohexylmethyl methacrylate (METHB) were studied to investigate water effects on conversion and polymer coating properties. METHB contains epoxide and methacrylate moieties, which undergo cationic and free-radical photopolymerization, respectively. The conversion of both groups was obtained by Raman spectroscopy in real time and depth. Water concentration and initiator system compositions were varied and shown to affect reaction kinetics and depth profile. With increasing water concentration, the epoxide induction period increased when only cationic initiator was present; however, the induction period disappeared when using the dual-initiator system. In addition, epoxide groups continued reacting after light was shuttered and reached a higher and more homogeneous conversion. Hybrid systems were shown to be less sensitive to water at low or intermediate concentrations. With high water concentrations, hybrid monomer systems manifested increased ring opening during illumination due to chain transfer and decreased physical properties due to loss of cross-linking.
Starch‐g‐poly(methyl methacrylate) (PMMA) was prepared by emulsion photopolymerization without ph... more Starch‐g‐poly(methyl methacrylate) (PMMA) was prepared by emulsion photopolymerization without photoinitiator. Grafting efficiency was determined for two types of starch (high‐amylose and amylopectin) and at several illumination times. Since clogging of the graft copolymer prevented vacuum filtration and Soxhlet extraction was too time‐consuming, a new method to separate the PMMA homopolymer from the graft copolymer was developed. Back‐flush filtration uses an intermittent pressure pulse to clean the filter as the homopolymer is separated from the graft copolymer. Back‐flush filtration was shown to be more efficient by reducing the separation time and solvent use, and the grafting efficiencies obtained with back‐flush filtration compare favorably with those from Soxhlet extraction for the starch‐g‐PMMA copolymer systems studied. More accurate grafting efficiencies could be obtained by applying a nonstick coating to the inside chamber of the back‐flush filtration unit.
C M-constant for chain transfer to the monomer DPPH-2,2-diphenyl-1-picrylhydrazyl EB-electron bea... more C M-constant for chain transfer to the monomer DPPH-2,2-diphenyl-1-picrylhydrazyl EB-electron beam G i-number of ions formed per 100 eV of energy absorbed by the system G(-m)-number of bonds broken per 100 eV of energy absorbed by the system G(M •)-number of initiating radicals formed per 100 eV of energy absorbed by the system G(R •)-number of primary radicals formed per 100 eV of energy absorbed by the system GPC-gel permeation chromatography k p-propagation rate constant k t-termination rate constant M •-initiating radical [M]-monomer concentration MMA-methyl methacrylate R p-rate of propagation R irate of initiation R R-rate of radical formation R •-primary radical X n-number-average degree of polymerization
Journal of Biomedical Materials Research Part A, Feb 2, 2010
Penetration of adhesives into the demineralized dentin surface and their subsequent conversion ar... more Penetration of adhesives into the demineralized dentin surface and their subsequent conversion are critically important to longevity of the adhesive resin-dentin bond. The durability of the resindentin bond is investigated by monitoring the change of adhesive concentration within the hybrid layer of aged specimens using Raman spectroscopy. Absolute molar concentrations of Bis-GMA and HEMA were measured across the hybrid layer of resin-dentin specimens 24 h after photopolymerization and after 24-wk storage in one of three media: artificial saliva (SAL), SAL containing cholesterol esterase to attack resin (EST), and SAL containing bacterial collagenase to attack collagen (COL). No significant difference among these groups for both Bis-GMA and HEMA molar concentrations at 24-h storage was found; however, concentrations decreased from the adhesive resin to the middle of the hybrid layer. Concentrations remained unchanged at any resin-dentin position after aging in SAL. In the hybrid layer, concentrations significantly decreased with aging in COL and tended to decrease in EST. While showing potential enzymatic biodegradative effects of endogenous matrix metalloproteinases and salivary esterases, this methodology may also prove to be a valuable assessment of new chemistries and future approaches to improve resin-dentin bond performance.
Photopolymerization systems based on hybrid monomer 3,4-epoxy-cyclohexylmethyl methacrylate (METH... more Photopolymerization systems based on hybrid monomer 3,4-epoxy-cyclohexylmethyl methacrylate (METHB) were studied to investigate the oxygen inhibition effects on the conversion and polymer properties in films and coatings. METHB contains epoxide and methacrylate functional groups, which undergo cationic and free-radical photopolymerization, respectively. The conversion of both groups was obtained by Raman confocal microscopy as a function of depth. Initiator system compositions were varied and shown to affect the depth profile. The methacrylate group conversion was low at the surface due to oxygen inhibition. When both reactions were present, a cross-linked network formed and reduced oxygen sensitivity. At depths greater than the oxygen-diffusion-affected region, both functional groups' conversions did not show depth dependence. In addition, epoxide groups continued reacting after light was shuttered and reached a higher and more homogeneous conversion. These systems exhibit lower sensitivity to oxygen and offer advantages such as increased cure speed and improved film-forming properties compared to free-radical systems.
poxy resins were cured via UV illumination in atmospheres of varying humidity levels. The chain-t... more poxy resins were cured via UV illumination in atmospheres of varying humidity levels. The chain-transfer reaction between the epoxy moieties and water molecules is described by the activated monomer mechanism. The physical effects of chain-transfer reactions mainly occur at the surface due to limited diffusion during the reaction. Force curves were obtained via atomic-force microscopy and used to determine the surface modulus of cured resins. A decrease in modulus was observed for resins cured in humid atmospheres.
This study examined the adhesive interface created by Filtek Silorane Adhesive System (SIL, 3M ES... more This study examined the adhesive interface created by Filtek Silorane Adhesive System (SIL, 3M ESPE, USA) using micro-Raman spectroscopy. The HL thickness of the adhesive interface created by SIL was comparable with data previously reported on self-etch adhesives, while the adhesive layer was considerably thicker. As previous investigations revealed that high DC is a fundamental pre-requisite for the stability of the resin bond over time, the DC results of the present study support the hypothesis that optimal stability of SIL within the HL can be obtained. However, further studies are needed to investigate the mechanical properties of the hybrid layer created by SIL and its stability over time
Although cationic photopolymerizations are not inhibited by oxygen, polymerization rates and conv... more Although cationic photopolymerizations are not inhibited by oxygen, polymerization rates and conversions of commonly used cycloaliphatic epoxides are typically much lower than industrially prevalent acrylates undergoing free-radical photopolymerization. The addition...
Composites Part A-applied Science and Manufacturing, Jun 1, 2013
The use of thick sections of fiber-reinforced polymers (FRPs) is increasing for numerous industri... more The use of thick sections of fiber-reinforced polymers (FRPs) is increasing for numerous industrial applications such as wind turbine blades. In situ cure monitoring is very important to directly observe the cure process of FRPs during the manufacturing process. In this work, Raman spectroscopy and dielectric analysis (DEA) are investigated for in situ cure monitoring of an epoxy resin. The cure behavior is first characterized using differential scanning calorimetry (DSC) as a baseline comparison, and the best-fit phenomenological reaction model is determined to describe the cure behavior of the epoxy resin as well as the kinetic parameters. The relationship between T g and degree of cure is also established. The degree of cure obtained from Raman spectroscopy and DEA under isothermal conditions is compared to that obtained from DSC. A good agreement is observed among the three methods, supporting the potential of these in situ cure monitoring methods during manufacturing. An implementation plan for in-plant monitoring is also discussed.
Flipping is an appealing method to engage students for meaningful and active learning. However, t... more Flipping is an appealing method to engage students for meaningful and active learning. However, the work needed to generate the content the students will use outside the classroom and to coordinate the activities the students will do inside the classroom seems daunting. This paper provides ideas to reduce the start-up costs in time and energy, essential elements to include in the implementation of the flip, and instructor and student behaviors that may need adjusting for a more effective flipped experience. These helpful hints are based on the author's experience in flipping her sophomore-level chemical engineering required course for the first time last fall.
Electron-beam (EB) and photo-polymerization rely on radiation chemistry; however, fundamental dif... more Electron-beam (EB) and photo-polymerization rely on radiation chemistry; however, fundamental differences arise in initiation and energy deposition. Since photopolymerization has been widely studied, a quantitative comparison would facilitate a better understanding of the kinetic and physical impact of the EB initiation mechanism, which is substantially more difficult to characterize. Here, a protocol was developed that enables investigation of EB and photopolymerized films of equivalent initiation energies. Using this protocol, the impact of the initiation mechanism on energy rate effects (EREs) was characterized for a series of five acrylate monomers. Differences in polymer conversion and physical properties were determined via Raman spectroscopy and dynamic mechanical analysis, respectively. In comparison to the EBpolymerized films of equivalent initiation energy and energy rate, the photopolymerized films had equal or lower conversions. Additionally, monomer chemistry was a key factor for differentiating ERE magnitude across the five-monomer series for both initiation mechanisms. Differences between initiation mechanisms were also demonstrated in T g , FWHM of the tan δ peak, and the maximum tan δ peak height for two of the monomer formulations. Quantifying these differences not only improves knowledge of fundamental kinetics, but also provides a foundation for predictive chemical structure/processing parameter/polymer property relationships.
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Papers by Julie Jessop