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Kashmari K, Patil SU, Kemppainen J, Shankara G, Odegard GM. Optimal Molecular Dynamics System Size for Increased Precision and Efficiency for Epoxy Materials. J Phys Chem B 2024; 128:4255-4265. [PMID: 38648370 DOI: 10.1021/acs.jpcb.4c00845] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 04/25/2024]
Abstract
Molecular dynamics (MD) simulation is an important tool for predicting thermo-mechanical properties of polymer resins at the nanometer length scale, which is particularly important for efficient computationally driven design of advanced composite materials and structures. Because of the statistical nature of modeling amorphous materials on the nanometer length scale, multiple MD models (replicates) are typically built and simulated for statistical sampling of predicted properties. Larger replicates generally provide higher precision in the predictions but result in higher simulation times. Unfortunately, there is insufficient information in the literature to establish guidelines between MD model size and the resulting precision in predicted thermo-mechanical properties. The objective of this study was to determine the optimal MD model size of epoxy resin to balance efficiency and precision. The results show that an MD model size of 15,000 atoms provides for the fastest simulations without sacrificing precision in the prediction of mass density, elastic properties, strength, and thermal properties of epoxy. The results of this study are important for efficient computational process modeling and integrated computational materials engineering (ICME) for the design of next-generation composite materials for demanding applications.
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Affiliation(s)
- Khatereh Kashmari
- Michigan Technological University, Houghton, Michigan 49931, United States
| | - Sagar U Patil
- Michigan Technological University, Houghton, Michigan 49931, United States
| | - Josh Kemppainen
- Michigan Technological University, Houghton, Michigan 49931, United States
| | - Gowtham Shankara
- Michigan Technological University, Houghton, Michigan 49931, United States
| | - Gregory M Odegard
- Michigan Technological University, Houghton, Michigan 49931, United States
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2
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Prasad D, Mitra N. Noncovalent Interactions in Mechanical Response of Thermoset Epoxy Resin. J Phys Chem B 2024. [PMID: 38422510 DOI: 10.1021/acs.jpcb.3c07369] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 03/02/2024]
Abstract
Free volume in polymers is known to influence the mechanical response of the polymers. Noncovalent interactions such as hydrogen bonds, Coulombic electrostatic interactions, and van der Waals interactions are present within these free volume regions. The manuscript presents a comprehensive identification, characterization, and evolution of noncovalent interactions as a thermoset epoxy resin (typically used as an interfacial adhesive material) is subjected to uniaxial tension, shear, and shock loading. Even though noncovalent interactions dominate uniaxial tension and shear response (up to strain levels of 50% wherein covalent bond dissociation is not observed), both covalent and noncovalent interactions define response for shock loading. Van der Waals interactions dominate the response as the samples are subjected to strain levels of 50% in tension and shear. In contrast, hydrogen bonds influence shock response.
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Affiliation(s)
- Dipak Prasad
- Hopkins Extreme Materials Institute and Department of Mechanical Engineering, Johns Hopkins University, Baltimore, Maryland 21218, United States
| | - Nilanjan Mitra
- Hopkins Extreme Materials Institute and Department of Mechanical Engineering, Johns Hopkins University, Baltimore, Maryland 21218, United States
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3
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Rao W, Tao J, Yang F, Wu T, Yu C, Zhao HB. Growth of copper organophosphate nanosheets on graphene oxide to improve fire safety and mechanical strength of epoxy resins. CHEMOSPHERE 2023; 311:137047. [PMID: 36336017 DOI: 10.1016/j.chemosphere.2022.137047] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/24/2022] [Revised: 08/04/2022] [Accepted: 10/26/2022] [Indexed: 06/16/2023]
Abstract
With the high integration of electronic products in our daily life, high-performance epoxy resins (EP) with excellent flame retardancy, smoke suppression, and mechanical strength are highly desired for applications. In this study, copper organophosphate nanosheets were evenly grown on the surface of graphene oxide (GO) via a self-assembly process based on coordination bonding and electrostatic interactions. The resultant nanohybrid endowed EP with satisfactory flame retardant effect and improved mechanical properties. Incorporating functionalized nanosheets of merely 1 wt% loading, the impact strength of the EP nanocomposites improved by 147% when compared to 1% EP-GO. Additionally, the nanosheets inhibited the smoke and heat release of EP, and the limiting oxygen value of EP-EGOPC reached ∼29%. The mechanism analysis verified that the existence of organophosphate and copper-containing components associated with the physical barrier of GO promoted the hybrid aromatization of the char layer, thereby improving the fire safety of epoxy matrix. This research offers a new interfacial method for designing functional nanosheets with good interface compatibility and high flame-retardant efficiency in polymers.
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Affiliation(s)
- Wenhui Rao
- Key Laboratory of New Processing Technology for Nonferrous Metals and Materials, Ministry of Education, Guangxi Key Laboratory of Optical and Electronic Materials and Devices, College of Materials Science and Engineering, Guilin University of Technology (GUT), Guilin, 541004, China
| | - Jie Tao
- Key Laboratory of New Processing Technology for Nonferrous Metals and Materials, Ministry of Education, Guangxi Key Laboratory of Optical and Electronic Materials and Devices, College of Materials Science and Engineering, Guilin University of Technology (GUT), Guilin, 541004, China
| | - Feihao Yang
- Key Laboratory of New Processing Technology for Nonferrous Metals and Materials, Ministry of Education, Guangxi Key Laboratory of Optical and Electronic Materials and Devices, College of Materials Science and Engineering, Guilin University of Technology (GUT), Guilin, 541004, China
| | - Tao Wu
- Key Laboratory of New Processing Technology for Nonferrous Metals and Materials, Ministry of Education, Guangxi Key Laboratory of Optical and Electronic Materials and Devices, College of Materials Science and Engineering, Guilin University of Technology (GUT), Guilin, 541004, China
| | - Chuanbai Yu
- Key Laboratory of New Processing Technology for Nonferrous Metals and Materials, Ministry of Education, Guangxi Key Laboratory of Optical and Electronic Materials and Devices, College of Materials Science and Engineering, Guilin University of Technology (GUT), Guilin, 541004, China.
| | - Hai-Bo Zhao
- The Collaborative Innovation Center for Eco-Friendly and Fire-Safety Polymeric Materials (MoE), National Engineering Laboratory of Eco-Friendly Polymeric Materials (Sichuan), College of Chemistry, Sichuan University, Chengdu, 610064, China.
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4
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Cozza RC, Verma V. Evaluation of fracture toughness of epoxy polymer composite incorporating micro/nano silica, rubber and CNTs. POLIMEROS 2020. [DOI: 10.1590/0104-1428.05720] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Affiliation(s)
- Ronaldo Câmara Cozza
- Centro Universitário da Fundação Educacional Inaciana “Padre Sabóia de Medeiros”, Brasil
| | - Vikas Verma
- National University of Science and Technology, Russia
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5
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Reghunadhan A, Datta J, Jaroszewski M, Kalarikkal N, Thomas S. Polyurethane glycolysate from industrial waste recycling to develop low dielectric constant, thermally stable materials suitable for the electronics. ARAB J CHEM 2020. [DOI: 10.1016/j.arabjc.2018.03.012] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/17/2022] Open
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6
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Kam KW, Teh PL, Osman H, Yeoh CK. Characterization of different forms of vulcanized natural rubbers as elastomer spacer and toughening agent in two-matrix filled epoxy/natural rubber/graphene nano-platelets system. J Appl Polym Sci 2018. [DOI: 10.1002/app.47198] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023]
Affiliation(s)
- Ka Wei Kam
- School of Materials Engineering, Kompleks Pusat Pengajian Jejawi 2, Taman Muhibbah; Universiti Malaysia Perlis; 02600, Jejawi Arau Perlis Malaysia
| | - Pei Leng Teh
- School of Materials Engineering, Kompleks Pusat Pengajian Jejawi 2, Taman Muhibbah; Universiti Malaysia Perlis; 02600, Jejawi Arau Perlis Malaysia
| | - Hakimah Osman
- School of Materials Engineering, Kompleks Pusat Pengajian Jejawi 2, Taman Muhibbah; Universiti Malaysia Perlis; 02600, Jejawi Arau Perlis Malaysia
| | - Cheow Keat Yeoh
- School of Materials Engineering, Kompleks Pusat Pengajian Jejawi 2, Taman Muhibbah; Universiti Malaysia Perlis; 02600, Jejawi Arau Perlis Malaysia
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7
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Wu X, Yang X, Yu R, Zhao XJ, Zhang Y, Huang W. Highly crosslinked and uniform thermoset epoxy microspheres: Preparation and toughening study. POLYMER 2018. [DOI: 10.1016/j.polymer.2018.04.011] [Citation(s) in RCA: 20] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/17/2022]
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8
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Marouf BT, Mai YW, Bagheri R, Pearson RA. Toughening of Epoxy Nanocomposites: Nano and Hybrid Effects. POLYM REV 2016. [DOI: 10.1080/15583724.2015.1086368] [Citation(s) in RCA: 86] [Impact Index Per Article: 10.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/22/2022]
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9
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Yu JW, Jung J, Choi YM, Choi JH, Yu J, Lee JK, You NH, Goh M. Enhancement of the crosslink density, glass transition temperature, and strength of epoxy resin by using functionalized graphene oxide co-curing agents. Polym Chem 2016. [DOI: 10.1039/c5py01483b] [Citation(s) in RCA: 83] [Impact Index Per Article: 10.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Epoxy nanocomposites are fabricated by using diamine-functionalized GO and exhibit high Tg, tensile strength, and crosslink density.
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Affiliation(s)
- Jin Won Yu
- Carbon Composite Materials Research Center
- Institute of Advanced Composites Materials
- Korea Institute of Science and Technology (KIST)
- Wanju-gun
- Korea
| | - Jin Jung
- Carbon Composite Materials Research Center
- Institute of Advanced Composites Materials
- Korea Institute of Science and Technology (KIST)
- Wanju-gun
- Korea
| | - Yong-Mun Choi
- Carbon Composite Materials Research Center
- Institute of Advanced Composites Materials
- Korea Institute of Science and Technology (KIST)
- Wanju-gun
- Korea
| | - Jae Hun Choi
- Carbon Composite Materials Research Center
- Institute of Advanced Composites Materials
- Korea Institute of Science and Technology (KIST)
- Wanju-gun
- Korea
| | - Jaesang Yu
- Carbon Composite Materials Research Center
- Institute of Advanced Composites Materials
- Korea Institute of Science and Technology (KIST)
- Wanju-gun
- Korea
| | - Jae Kwan Lee
- Department of Carbon Materials
- Chosun University
- Gwangju
- Korea
| | - Nam-Ho You
- Carbon Composite Materials Research Center
- Institute of Advanced Composites Materials
- Korea Institute of Science and Technology (KIST)
- Wanju-gun
- Korea
| | - Munju Goh
- Carbon Composite Materials Research Center
- Institute of Advanced Composites Materials
- Korea Institute of Science and Technology (KIST)
- Wanju-gun
- Korea
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10
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Comparative investigation of thermal and mechanical properties of cross-linked epoxy polymers with different curing agents by molecular dynamics simulation. J Mol Graph Model 2015; 62:157-164. [PMID: 26432014 DOI: 10.1016/j.jmgm.2015.09.012] [Citation(s) in RCA: 38] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/20/2015] [Revised: 09/13/2015] [Accepted: 09/14/2015] [Indexed: 11/21/2022]
Abstract
Molecular dynamics (MD) simulations were carried out to predict the thermal and mechanical properties of the cross-linked epoxy system composed of DGEBA resin and the curing agent TETA. To investigate the effects of curing agents, a comprehensive and comparative study was also performed on the thermal and mechanical properties of DGEBA/TETA and DGEBA/DETDA epoxy systems such as density, glass transition temperature (Tg), coefficient of thermal expansion (CTE) and elastic properties of different cross-linking densities and different temperatures. The results indicated that the glass transition temperature of DGEBA/TETA system calculated through density-temperature data, ∼ 385-395 °K, for the epoxy system with the cross-linking density of 62.5% has a better agreement with the experimental value (Tg, ∼ 400 °K) in comparison to the value calculated through the variation of cell volume in terms of temperature, 430-440 °K. They also indicated that CTE related parameters and elastic properties including Young, Bulk, and shear's moduli, and Poisson's ratio have a relative agreement with the experimental results. Comparison between the thermal and mechanical properties of epoxy systems of DGEBA/TETA and DGEBA/DETDA showed that the DGEBA/DETDA has a higher Tg in all cross linking densities than that of DGEBA/TETA, while higher mechanical properties was observed in the case of DGEBA/TETA in almost all cross linking densities.
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11
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Angelsky OV, Bekshaev AY, Maksimyak PP, Maksimyak AP, Hanson SG. Measurement of small light absorption in microparticles by means of optically induced rotation. OPTICS EXPRESS 2015; 23:7152-7163. [PMID: 25837060 DOI: 10.1364/oe.23.007152] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/04/2023]
Abstract
The absorption parameters of micro-particles have been associated with the induced spin exerted upon the particle, when embedded in a circularly polarized coherent field. The induced rotational speed is theoretically analyzed, showing the influence of the beam parameters, the parameters of the particle and the tribological parameters of the surrounding fluid. The theoretical findings have been adequately confirmed in experiments.
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12
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Park SJ, Heo GY, Jin FL, Rhee KY. Effect of urethane functionality and number of epoxide groups on cure and mechanical behaviors of epoxy resins. Macromol Res 2015. [DOI: 10.1007/s13233-015-3015-3] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/24/2022]
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13
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Gavrilov AA, Komarov PV, Khalatur PG. Thermal Properties and Topology of Epoxy Networks: A Multiscale Simulation Methodology. Macromolecules 2014. [DOI: 10.1021/ma502220k] [Citation(s) in RCA: 65] [Impact Index Per Article: 6.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/02/2023]
Affiliation(s)
- Alexey A. Gavrilov
- Physics
Department, Lomonosov Moscow State University, Moscow 119991, Russia
- Institute
for Advanced Energy Related Nanomaterials, University of Ulm, Ulm D-89069, Germany
| | - Pavel V. Komarov
- Institute
of Organoelement Compounds, Russian Academy of Science, Moscow 119991, Russia
- Department
of Theoretical Physics, Tver State University, Tver 170002, Russia
| | - Pavel G. Khalatur
- Institute
for Advanced Energy Related Nanomaterials, University of Ulm, Ulm D-89069, Germany
- Institute
of Organoelement Compounds, Russian Academy of Science, Moscow 119991, Russia
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14
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Xin D, Han Q. Study on thermomechanical properties of cross-linked epoxy resin. MOLECULAR SIMULATION 2014. [DOI: 10.1080/08927022.2014.938334] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/25/2022]
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15
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Guo T, Zhou Z, Guo H, Xiao G, Tang X, Peng M. Toughening of epoxy resin with functionalized core-sheath structured PAN/SBS electrospun fibers. J Appl Polym Sci 2014. [DOI: 10.1002/app.41119] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022]
Affiliation(s)
- Tanghua Guo
- MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering; Zhejiang University; Hangzhou 310027 China
| | - Zhi Zhou
- MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering; Zhejiang University; Hangzhou 310027 China
| | - Honglei Guo
- MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering; Zhejiang University; Hangzhou 310027 China
| | - Guohua Xiao
- MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering; Zhejiang University; Hangzhou 310027 China
| | - Xinglei Tang
- MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering; Zhejiang University; Hangzhou 310027 China
| | - Mao Peng
- MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering; Zhejiang University; Hangzhou 310027 China
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16
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Affiliation(s)
- Chunyu Li
- Department of Materials Engineering and Birck Nanotechnology Center, Purdue University; West Lafayette Indiana 47906
| | - Alejandro Strachan
- Department of Materials Engineering and Birck Nanotechnology Center, Purdue University; West Lafayette Indiana 47906
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17
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Developing a Sealing Material: Effect of Epoxy Modification on Specific Physical and Mechanical Properties. MATERIALS 2013; 6:5490-5501. [PMID: 28788404 PMCID: PMC5452763 DOI: 10.3390/ma6125490] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 09/30/2013] [Revised: 11/11/2013] [Accepted: 11/21/2013] [Indexed: 11/28/2022]
Abstract
To develop a matched sealing material for socket rehabilitation of grey cast iron pipes, an epoxy resin is modified by the addition of different components to improve the flexibility. Three different modifications are made by adding ethylene-propylene diene monomer (EPDM) rubber powder, reactive liquid polymer (ATBN) and epoxidized modifier. In this paper the effect of the modification method as well as the influence of absorption of water on the mechanical and physical properties are analyzed in terms of: tensile strength, modulus of elasticity, adhesion performance, pressure resistance, glass transition temperature and water content. A comparison with neat epoxy shows for all materials that the modulus of elasticity and strength decrease. Unlike other tested modification methods, the modification with rubber powder did not enhance the flexibility. All materials absorb water and a plasticization effect arises with further changes of mechanical and physical properties. The application of the sealant on the grey cast iron leads to a reduction of the strain at break (in comparison to the common tensile test of the pure materials) and has to be evaluated. The main requirement of pressure resistance up to 1 MPa was tested on two chosen materials. Both materials fulfill this requirement.
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18
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Molecular modeling of elastic properties of thermosetting polymers using a dynamic deformation approach. POLYMER 2013. [DOI: 10.1016/j.polymer.2013.04.034] [Citation(s) in RCA: 46] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
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19
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Shenogina NB, Tsige M, Patnaik SS, Mukhopadhyay SM. Molecular Modeling Approach to Prediction of Thermo-Mechanical Behavior of Thermoset Polymer Networks. Macromolecules 2012. [DOI: 10.1021/ma3007587] [Citation(s) in RCA: 109] [Impact Index Per Article: 9.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Affiliation(s)
- Natalia B. Shenogina
- Department
of Mechanical and
Materials Engineering, Wright State University, Dayton, Ohio, United States
| | - Mesfin Tsige
- Department of Polymer Science, University of Akron, Akron, Ohio, United States
| | - Soumya S. Patnaik
- Propulsion Directorate, Air Force Research Laboratory, Dayton, Ohio, United
States
| | - Sharmila M. Mukhopadhyay
- Department
of Mechanical and
Materials Engineering, Wright State University, Dayton, Ohio, United States
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20
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Characterization of shape memory behaviour of CTBN-epoxy resin system. JOURNAL OF POLYMER RESEARCH 2012. [DOI: 10.1007/s10965-012-9894-5] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/28/2022]
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21
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Rungswang W, Kato K, Kotaki M, Chirachanchai S. Size-controllable nanospheres prepared by blending a thermoset monomer in confined morphology with thermoplastic elastomer. POLYMER 2012. [DOI: 10.1016/j.polymer.2012.02.002] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/14/2022]
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22
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23
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Cao Y, Shao Y, Sun J, Lin S. Mechanical properties of an epoxy resin toughened by polyester. J Appl Polym Sci 2003. [DOI: 10.1002/app.13007] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
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24
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Mamo A, Orestano A, Pollicino A, Recca A. Synthesis and Characterization of an Epoxy Ended Poly(Ether Sulphone)/Poly(Ether Ether Sulfphone) Copolymer. JOURNAL OF POLYMER ENGINEERING 2002. [DOI: 10.1515/polyeng.2002.22.3.209] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/15/2022]
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25
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Mulder FM, Jansen BJP, Lemstra PJ, Meijer HEH, de Groot HJM. Pronounced Poly(methyl methacrylate) Dynamics Induced by Blending Morphology. Macromolecules 2000. [DOI: 10.1021/ma9904619] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- F. M. Mulder
- Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, P. O. Box 9502, 2300 RA Leiden, The Netherlands, and Eindhoven University of Technology, P. O. Box 513, 5600 MB Eindhoven, The Netherlands
| | - B. J. P. Jansen
- Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, P. O. Box 9502, 2300 RA Leiden, The Netherlands, and Eindhoven University of Technology, P. O. Box 513, 5600 MB Eindhoven, The Netherlands
| | - P. J. Lemstra
- Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, P. O. Box 9502, 2300 RA Leiden, The Netherlands, and Eindhoven University of Technology, P. O. Box 513, 5600 MB Eindhoven, The Netherlands
| | - H. E. H. Meijer
- Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, P. O. Box 9502, 2300 RA Leiden, The Netherlands, and Eindhoven University of Technology, P. O. Box 513, 5600 MB Eindhoven, The Netherlands
| | - H. J. M. de Groot
- Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, P. O. Box 9502, 2300 RA Leiden, The Netherlands, and Eindhoven University of Technology, P. O. Box 513, 5600 MB Eindhoven, The Netherlands
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