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1.
Cureus ; 16(5): e59532, 2024 May.
Artigo em Inglês | MEDLINE | ID: mdl-38826951

RESUMO

Hypertension is a globally prevalent condition, and low adherence to antihypertensive therapy is considered one of the main causes of poor blood pressure (BP) control. Non-adherence to antihypertensive treatment is a complex issue that can arise from various factors; however, gaining an understanding of this provides key targets for intervention strategies. This study aimed to provide an overview of the current status and recent developments regarding our understanding of the determinants of patients' adherence to antihypertensives. A systematic review was performed using the electronic databases MEDLINE/PubMed, Web of Science, Scientific Electronic Library Online (SciELO), and "Índex das Revistas Médicas Portuguesas", which included studies published between 2017 and 2021 following the PICOS model: (P) Adult patients with the diagnosis of primary hypertension, using at least one antihypertensive agent; (I) all interventions on both pharmacological and non-pharmacological level; (C) patient's adherence against their non-adherence; (O) changes in adherence to the therapeutic plan; and (S) any study design (except review articles) written in English, French, Spanish or Portuguese. Articles were reviewed by two researchers and their quality was assessed. Subsequently, determinants were classified according to their consistent or inconsistent association with adherence or non-adherence. Only 45 of the 635 reports identified met the inclusion criteria. Adherence was consistently associated with patient satisfaction with communication, patient-provider relationship, their treatment, and use of eHealth and mHealth strategies; a patient's mental and physical health, including depression, cognitive impairment, frailty, and disability, previous hospitalization, occurrence of vital events; drug treatment type and appearance; and unwillingness due to health literacy, self-efficacy, and both implicit and explicit attitudes towards treatment. There were discrepancies regarding the association of other factors to adherence, but these inconsistent factors should also be taken into account. In conclusion, the barriers to adherence are varied and often interconnected between socioeconomic, patient, therapy, condition, and healthcare system levels. Healthcare teams should invest in studying patients' non-adherence motives and tailoring interventions to individual levels, by using a multifaceted approach to assess adherence. Further research is needed to analyze the impact of implicit attitudes, the use of new technological approaches, and the influence of factors that are inconsistently associated with non-adherence, to understand their potential in implementing adherence strategies.

2.
Arch Med Sci ; 18(6): 1498-1504, 2022.
Artigo em Inglês | MEDLINE | ID: mdl-36457974

RESUMO

Introduction: Given the number of patients with mental conditions who receive treatment within the primary care (PC) context, and the high prevalence of multimorbidity (especially in older people), there is a need to study mental-physical multimorbidity (MPM) in this population and context. This study sought to identify the impact on health-related quality of life (QoL) of MPM in adults aged 60 years and older. Material and methods: Secondary analysis of data derived from 251 primary health individuals. Data were collected via a sociodemographic and clinical questionnaire. Health-related QoL was assessed using the SF-12 instrument. Multiple linear regressions were performed for physical and mental health in MPM patients and in patients with physical-only multimorbidity. Results: Mean age of participants was 70.6 years; 57.8% were female. Quality of life was lower in MPM patients than in those with physical-only multimorbidity. Regarding MPM patients, female sex, 75 years and over, and low income were associated with worse physical health. Female sex was also associated with worse mental health. Conclusions: This study contributes to the global knowledge of MPM in older people, illuminates health-related QoL differences among MPM and physical-only multimorbidity patients, and highlights the importance of non-modifiable characteristics associated with deterioration of health-related QoL. Team collaboration between primary care physicians, psychiatrists (and other mental health providers), and social workers may be necessary to assess psychiatric and physical symptoms and provide for the care needs of older people with MPM.

3.
Sci Rep ; 11(1): 3449, 2021 02 10.
Artigo em Inglês | MEDLINE | ID: mdl-33568680

RESUMO

Instrumented implants are being developed with a radically innovative design to significantly reduce revision surgeries. Although bone replacements are among the most prevalent surgeries performed worldwide, implant failure rate usually surpasses 10%. High sophisticated multifunctional bioelectronic implants are being researched to incorporate cosurface capacitive architectures with ability to deliver personalized electric stimuli to peri-implant target tissues. However, the ability of these architectures to detect bone-implant interface states has never been explored. Moreover, although more than forty technologies were already proposed to detect implant loosening, none is able to ensure effective monitoring of the bone-implant debonding, mainly during the early stages of loosening. This work shows, for the first time, that cosurface capacitive sensors are a promising technology to provide an effective monitoring of bone-implant interfaces during the daily living of patients. Indeed, in vitro experimental tests and simulation with computational models highlight that both striped and circular capacitive architectures are able to detect micro-scale and macro-scale interface bonding, debonding or loosening, mainly when bonding is weakening or loosening is occurring. The proposed cosurface technologies hold potential to implement highly effective and personalized sensing systems such that the performance of multifunctional bioelectronic implants can be strongly improved. Findings were reported open a new research line on sensing technologies for bioelectronic implants, which may conduct to great impacts in the coming years.

4.
Sensors (Basel) ; 20(1)2019 Dec 23.
Artigo em Inglês | MEDLINE | ID: mdl-31878028

RESUMO

Musculoskeletal disorders are becoming an ever-growing societal burden and, as a result, millions of bone replacements surgeries are performed per year worldwide. Despite total joint replacements being recognized among the most successful surgeries of the last century, implant failure rates exceeding 10% are still reported. These numbers highlight the necessity of technologies to provide an accurate monitoring of the bone-implant interface state. This study provides a detailed review of the most relevant methodologies and technologies already proposed to monitor the loosening states of endoprosthetic implants, as well as their performance and experimental validation. A total of forty-two papers describing both intracorporeal and extracorporeal technologies for cemented or cementless fixation were thoroughly analyzed. Thirty-eight technologies were identified, which are categorized into five methodologies: vibrometric, acoustic, bioelectric impedance, magnetic induction, and strain. Research efforts were mainly focused on vibrometric and acoustic technologies. Differently, approaches based on bioelectric impedance, magnetic induction and strain have been less explored. Although most technologies are noninvasive and are able to monitor different loosening stages of endoprosthetic implants, they are not able to provide effective monitoring during daily living of patients.


Assuntos
Transplante Ósseo/métodos , Falha de Prótese , Humanos , Magnetismo , Som , Vibração
5.
Cureus ; 11(6): e4990, 2019 Jun 25.
Artigo em Inglês | MEDLINE | ID: mdl-31497421

RESUMO

BACKGROUND: Multimorbidity brings several difficulties and challenges to the daily work of primary care teams. Team meetings are opportunities to discuss approaches and solutions on how to best manage multimorbid patients. OBJECTIVE: This qualitative study aimed to collect a consensus, from general practitioners that deal with multimorbid patients, about their perspectives regarding multimorbidity team meetings in primary care. METHODS: The study followed a modified Delphi method with 15 Portuguese general practitioners. After every round of responses, results were analyzed, and justifications for non-consensual items were aggregated by the investigators, and then a new Delphi round with the revised questionnaire was again initiated. This process was repeated until consensus has been reached. RESULTS: Overall, a list of 10 key themes associated with the ideal meeting was agreed: (a) definition; (b) setting; (c) duration; (d) frequency; (e) number of participants; (f) attendance; (g) requirement of patient's presence; (h) number of patients/clinical cases; (i) structure of the meeting; and (j) sharing meeting results. The consensus was achieved after two Delphi rounds with a mean score between 7.9 and 8.7 (maximum score of 9.0 per key theme). CONCLUSION: The complexity of multimorbidity affects meetings' periodicity, duration, and participants. Ideally, it should be an interprofessional primary care team meeting. Further research exploring meeting outcomes (organizational effectiveness and healthcare quality) of the proposed factors is needed before they can be recommended for general use.

6.
Sci Rep ; 9(1): 5001, 2019 03 21.
Artigo em Inglês | MEDLINE | ID: mdl-30899061

RESUMO

Cosurface electrode architectures are able to deliver personalized electric stimuli to target tissues. As such, this technology holds potential for a variety of innovative biomedical devices. However, to date, no detailed analyses have been conducted to evaluate the impact of stimulator architecture and geometry on stimuli features. This work characterizes, for the first time, the electric stimuli delivered to bone cellular tissues during in vitro experiments, when using three capacitive architectures: stripped, interdigitated and circular patterns. Computational models are presented that predict the influence of cell confluence, cosurface architecture, electrodes geometry, gap size between electrodes and power excitation on the stimuli delivered to cellular layers. The results demonstrate that these stimulators are able to deliver osteoconductive stimuli. Significant differences in stimuli distributions were observed for different stimulator designs and different external excitations. The thickness specification was found to be of utmost importance. In vitro experiments using an osteoblastic cell line highlight that cosurface stimulation at a low frequency can enhance osteoconductive responses, with some electrode-specific differences being found. A major feature of this type of work is that it enables future detailed analyses of stimuli distribution throughout more complex biological structures, such as tissues and organs, towards sophisticated biodevice personalization.


Assuntos
Simulação por Computador , Estimulação Elétrica/instrumentação , Próteses e Implantes , Eletrodos , Humanos , Medicina de Precisão
7.
Sci Rep ; 6: 30231, 2016 07 26.
Artigo em Inglês | MEDLINE | ID: mdl-27456818

RESUMO

Non-drug strategies based on biophysical stimulation have been emphasized for the treatment and prevention of musculoskeletal conditions. However, to date, an effective stimulation system for intracorporeal therapies has not been proposed. This is particularly true for active intramedullary implants that aim to optimize osseointegration. The increasing demand for these implants, particularly for hip and knee replacements, has driven the design of innovative stimulation systems that are effective in bone-implant integration. In this paper, a new cosurface-based capacitive system concept is proposed for the design of implantable devices that deliver controllable and personalized electric field stimuli to target tissues. A prototype architecture of this system was constructed for in vitro tests, and its ability to deliver controllable stimuli was numerically analyzed. Successful results were obtained for osteoblastic proliferation and differentiation in the in vitro tests. This work provides, for the first time, a design of a stimulation system that can be embedded in active implantable devices for controllable bone-implant integration and regeneration. The proposed cosurface design holds potential for the implementation of novel and innovative personalized stimulatory therapies based on the delivery of electric fields to bone cells.


Assuntos
Desenvolvimento Ósseo/fisiologia , Fixação Intramedular de Fraturas/métodos , Osseointegração/efeitos da radiação , Próteses e Implantes , Desenvolvimento Ósseo/efeitos da radiação , Diferenciação Celular/efeitos da radiação , Proliferação de Células/efeitos da radiação , Estimulação Elétrica , Humanos , Osseointegração/fisiologia , Propriedades de Superfície
8.
Sci Rep ; 6: 18579, 2016 Jan 04.
Artigo em Inglês | MEDLINE | ID: mdl-26725842

RESUMO

Magnetic levitation has been used to implement low-cost and maintenance-free electromagnetic energy harvesting. The ability of levitation-based harvesting systems to operate autonomously for long periods of time makes them well-suited for self-powering a broad range of technologies. In this paper, a combined theoretical and experimental study is presented of a harvester configuration that utilizes the motion of a levitated hard-magnetic element to generate electrical power. A semi-analytical, non-linear model is introduced that enables accurate and efficient analysis of energy transduction. The model predicts the transient and steady-state response of the harvester a function of its motion (amplitude and frequency) and load impedance. Very good agreement is obtained between simulation and experiment with energy errors lower than 14.15% (mean absolute percentage error of 6.02%) and cross-correlations higher than 86%. The model provides unique insight into fundamental mechanisms of energy transduction and enables the geometric optimization of harvesters prior to fabrication and the rational design of intelligent energy harvesters.

9.
J Phys Chem A ; 119(12): 2998-3007, 2015 Mar 26.
Artigo em Inglês | MEDLINE | ID: mdl-25734572

RESUMO

The gas-phase enthalpy of formation of cubane (603.4 ± 4 kJ mol(-1)) was calculated using an explicitly correlated composite method (W1-F12). The result obtained for cubane, together with the experimental value for the enthalpy of sublimation, 54.8 ± 2.0 kJ mol(-1), led to 548.6 ± 4.5 kJ mol(-1) for the solid-phase enthalpy of formation. This value is only 6.8 kJ mol(-1) higher than the 50-year-old original calorimetric result. The carbon-hydrogen bond dissociation enthalpy (C-H BDE) of cubane (438.4 ± 4 kJ mol(-1)), together with properties relevant for its experimental determination using gas-phase ion thermochemistry, namely the cubane gas-phase acidity (1704.6 ± 4 kJ mol(-1)), cubyl radical electron affinity (45.8 ± 4 kJ mol(-1)), cubane ionization energy (1435.1 ± 4 kJ mol(-1)), cubyl radical cation proton affinity (918.8 ± 4 kJ mol(-1)), cubane cation appearance energy (1099.6 ± 4 kJ mol(-1)), and cubyl ionization energy (661.2 ± 4 kJ mol(-1)), were also determined. These values were compared with those calculated for unstrained hydrocarbons (viz., methane, ethane, and isobutane). The strain energy of cubane (667.2 kJ mol(-1)) and cubyl radical (689.4 kJ mol(-1)) were independently estimated via quasihomodesmotic reactions. These values were related via a simple model to the C-H BDE in cubane. Taking into account the accuracy of the computational method, the comparison with high-precision experimental results, and the data consistency afforded by the relevant thermodynamic cycles, we claim an uncertainty better than ±4 kJ mol(-1) for the new enthalpy of formation values presented.


Assuntos
Ciclo-Octanos/análogos & derivados , Ciclo-Octanos/química , Ciclo-Octanos/síntese química , Teoria Quântica , Termodinâmica
10.
Comput Methods Biomech Biomed Engin ; 18(10): 1090-1098, 2015 Aug.
Artigo em Inglês | MEDLINE | ID: mdl-24460149

RESUMO

Isolated patellofemoral (PF) arthritis of the knee is a common cause of anterior knee pain and disability. Patellofemoral arthroplasty (PFA) is a bone conserving solution for patients with PF degeneration. Failure mechanisms of PFA include growing tibiofemoral arthritis and loosening of components. The implant loosening can be associated with bone resorption or fatigue-failure of bone by overload. This research work aims at determining the structural effects of the implantation of PF prosthesis Journey PFJ (Smith & Nephew, Inc., Memphis, TN, USA) on femoral cancellous bone. For this purpose, the finite element method is considered to perform computational simulations for different conditions, such as well-fixed and loosening scenarios. From the global results obtained, in the well-fixed scenario, a decrease in strain on cancellous bone was noticed, which can be related to bone resorption. In the loosening scenario, when the cement layer becomes inefficient, a significant increase in cancellous bone strain was observed, which can be associated with bone fatigue-failure.These strain changes suggest a weakness of the femur after PFA.

11.
Expert Rev Med Devices ; 11(6): 617-35, 2014 Nov.
Artigo em Inglês | MEDLINE | ID: mdl-25234709

RESUMO

This paper reviews instrumented hip joint replacements, instrumented femoral replacements and instrumented femoral fracture stabilizers. Examination of the evolution of such implants was carried out, including the detailed analysis of 16 architectures, designed by 8 research teams and implanted in 32 patients. Their power supply, measurement, communication, processing and actuation systems were reviewed, as were the tests carried out to evaluate their performance and safety. These instrumented implants were only designed to measure biomechanical and thermodynamic quantities in vivo, in order to use such data to conduct research projects and optimize rehabilitation processes. The most promising trend is to minimize aseptic loosening and/or infection following hip or femoral replacements or femoral stabilization procedures by using therapeutic actuators inside instrumented implants to apply controlled stimuli in the bone-implant interface.


Assuntos
Fraturas do Fêmur/diagnóstico , Fraturas do Fêmur/cirurgia , Fêmur/cirurgia , Prótese de Quadril , Fixadores Internos , Monitorização Ambulatorial/instrumentação , Tecnologia sem Fio/instrumentação , Eletrônica Médica/instrumentação , Desenho de Equipamento , Análise de Falha de Equipamento , Humanos , Avaliação da Tecnologia Biomédica
12.
J Biomech ; 46(15): 2561-71, 2013 Oct 18.
Artigo em Inglês | MEDLINE | ID: mdl-24050511

RESUMO

Instrumented hip implants were proposed as a method to monitor and predict the biomechanical and thermal environment surrounding such implants. Nowadays, they are being developed as active implants with the ability to prevent failures by loosening. The generation of electric energy to power active mechanisms of instrumented hip implants remains a question. Instrumented implants cannot be implemented without effective electric power systems. This paper surveys the power supply systems of seventeen implant architectures already implanted in-vivo, namely from instrumented hip joint replacements and instrumented fracture stabilizers. Only inductive power links and batteries were used in-vivo to power the implants. The energy harvesting systems, which were already designed to power instrumented hip implants, were also analyzed focusing their potential to overcome the disadvantages of both inductive-based and battery-based power supply systems. From comparative and critical analyses of the methods to power instrumented implants, one can conclude that: inductive powering and batteries constrain the full operation of instrumented implants; motion-driven electromagnetic energy harvesting is a promising method to power instrumented passive and active hip implants.


Assuntos
Fontes de Energia Elétrica , Prótese de Quadril , Humanos , Falha de Prótese
13.
J Biomed Opt ; 18(5): 50903, 2013 May.
Artigo em Inglês | MEDLINE | ID: mdl-23722494

RESUMO

As optical fibers revolutionize the way data is carried in telecommunications, the same is happening in the world of sensing. Fiber-optic sensors (FOS) rely on the principle of changing the properties of light that propagate in the fiber due to the effect of a specific physical or chemical parameter. We demonstrate the potentialities of this sensing concept to assess pressure in biomedical and biomechanical applications. FOSs are introduced after an overview of conventional sensors that are being used in the field. Pointing out their limitations, particularly as minimally invasive sensors, is also the starting point to argue FOSs are an alternative or a substitution technology. Even so, this technology will be more or less effective depending on the efforts to present more affordable turnkey solutions and peer-reviewed papers reporting in vivo experiments and clinical trials.


Assuntos
Tecnologia de Fibra Óptica/instrumentação , Monitorização Fisiológica/instrumentação , Animais , Fenômenos Biomecânicos , Pressão Sanguínea , Humanos , Pressão Intracraniana , Transdutores de Pressão
14.
J Phys Chem A ; 117(13): 2873-8, 2013 Apr 04.
Artigo em Inglês | MEDLINE | ID: mdl-23441731

RESUMO

The gas-phase enthalpies of formation for a set of ortho-substituted alkylbenzenes were obtained from CCSD(T*)-F12 and W1-F12 calculations. Most values are in keeping with available experimental data. The gas-phase enthalpies of formation of 1-ethyl-2-propylbenzene, 1-ethyl-2-isopropylbenzene, 1,2-diisopropylbenzene, 1,2,4-triethylbenzene, and 1,2,4,5-tetraethylbenzene, for which no experimental data are available, were determined as -46.0, -46.8, -68.7, -76.9, and -116.8 kJ mol(-1), respectively (estimated error bar ±4 kJ mol(-1)). The whole set of experimental and theoretical values are in good agreement with the estimates obtained using the extended Laidler bond additivity (ELBA) method. This agreement supports the approximation used in ELBA that interactions between ortho alkyl groups (other than tertiary alkyl groups) have roughly the same magnitude as a methyl-methyl interaction.

15.
Arch Gynecol Obstet ; 283(3): 501-8, 2011 Mar.
Artigo em Inglês | MEDLINE | ID: mdl-20237933

RESUMO

PURPOSE: To evaluate vaginal microflora and interleukin-1ß (IL-ß), interleukin-6 (IL-6), interleukin-8 (IL-8) and tumor necrosis factor-alpha (TNF-α) concentrations in the cervicovaginal fluid of a group of pregnant women in preterm labor when compared with a group of full-term pregnant women not yet in labor. METHOD: Case-control study performed in a University tertiary referral maternity in Campinas, Brazil with 45 pregnant women in preterm labor and 45 full-term pregnant women not in labor. All patients underwent speculum examination for the collection of cervicovaginal fluid. Bacterial vaginosis (BV) was diagnosed according to the criteria of Amsel and Nugent. Culture was performed for group B streptococcus (GBS) and lactobacilli, and hybrid capture assay for screening for chlamydial and gonococcal infection. Cytokine concentrations were measured using ELISA technique. Statistical analysis was performed using χ(2), Fisher's exact, and crude and adjusted odds ratios. Significance level was defined at 5%. The main outcome measures were cervicovaginal cytokines in preterm labor. RESULTS: IL-6 and IL-8 were significantly associated with preterm labor. The changes in vaginal microflora, as well as BV and GBS, were more frequent in women in preterm labor, although BV and GBS showed no statistical significance. The presence of Candida sp., absence of lactobacilli, positive screening for chlamydial and gonococcal infection and the presence of IL-1ß and TNF-α were not associated with preterm labor. CONCLUSIONS: IL-6 and IL-8 and the presence of any type of vaginal infection were the factors that were significantly associated with preterm labor.


Assuntos
Líquidos Corporais/imunologia , Citocinas/imunologia , Trabalho de Parto Prematuro/imunologia , Trabalho de Parto Prematuro/microbiologia , Complicações Infecciosas na Gravidez/microbiologia , Vagina/imunologia , Vagina/microbiologia , Adolescente , Adulto , Líquidos Corporais/química , Brasil , Estudos de Casos e Controles , Citocinas/análise , Feminino , Humanos , Inflamação/microbiologia , Interleucina-1beta/análise , Interleucina-1beta/imunologia , Interleucina-6/análise , Interleucina-6/imunologia , Interleucina-8/análise , Interleucina-8/imunologia , Gravidez , Fator de Necrose Tumoral alfa/análise , Fator de Necrose Tumoral alfa/imunologia , Vaginose Bacteriana/diagnóstico , Adulto Jovem
16.
J Phys Chem A ; 113(23): 6524-30, 2009 Jun 11.
Artigo em Inglês | MEDLINE | ID: mdl-19449831

RESUMO

The energetics of tert-butoxyl radical addition reaction to norbornadiene was investigated by time-resolved photoacoustic calorimetry (TR-PAC). The result, together with the C-O bond dissociation enthalpy (BDE) in the addition product, allowed us to calculate the pi-bond dissociation enthalpy in norbornadiene. Quantum chemistry (QC) methods were also used to obtain several enthalpies of reaction of the addition of oxygen-centered radicals to alkenes. The pi-bond dissociation enthalpies in these molecules were calculated by a procedure similar to that used in the case of norbornadiene and were compared with the pi-BDE values obtained by the method proposed by Benson. These two different approaches yield similar values for the pi-BDEs in alkenes, indicating that the addition method proposed in the present study is a valid way to derive that quantity. The influence of strain in the pi-BDEs of cyclic alkenes was investigated and allowed us to justify the difference between the pi-BDE in norbornene and norbornadiene. Finally, the thermochemistry of the addition and abstraction reactions involving these two molecules and tert-butoxyl radical was analyzed.


Assuntos
Carbono/química , Norbornanos/química , Óxidos/química , Alcenos/química , Radicais Livres/química , Oxigênio/química , Termodinâmica
17.
Virology ; 385(1): 22-7, 2009 Mar 01.
Artigo em Inglês | MEDLINE | ID: mdl-19117586

RESUMO

Women with bacterial vaginosis (BV) have a higher risk of HIV transmission but the cause of risk is unknown. Dendritic cells (DC) are implicated in transmission of HIV and we previously observed that DC mature when exposed to mucosal fluid from women with BV. We hypothesized that maturation of DC by BV mucosal fluid would enhance DC-mediated trans-infection of HIV. Monocyte-derived DC (MDDC) were treated with mucosal fluid, incubated with HIV(Bal), and HIV trans-infection was evaluated. While LPS-treated MDDC increased HIV(Bal)trans-infection, BV fluid reduced trans-infection. HIV(Bal) DNA levels in MDDC were not affected by BV fluid or LPS but productive infection of MDDC was decreased by LPS and BV fluid. Mucosal fluid from women with BV does not increase MDDC-mediated trans-infection suggesting that BV does not increase HIV susceptibility by increasing DC-mediated trans-infection. However, indirect effects of DC maturation on HIV transmission cannot be ruled out.


Assuntos
Células Dendríticas/virologia , Infecções por HIV/microbiologia , Infecções por HIV/virologia , HIV-1/fisiologia , Muco/metabolismo , Vaginose Bacteriana , Adjuvantes Imunológicos/farmacologia , Moléculas de Adesão Celular/imunologia , Células Dendríticas/efeitos dos fármacos , Células Dendríticas/imunologia , Feminino , Infecções por HIV/complicações , Infecções por HIV/imunologia , Infecções por HIV/transmissão , Humanos , Lipopolissacarídeos/farmacologia , Fatores de Tempo , Vaginose Bacteriana/complicações , Vaginose Bacteriana/imunologia
18.
J Nanosci Nanotechnol ; 8(8): 4008-12, 2008 Aug.
Artigo em Inglês | MEDLINE | ID: mdl-19049168

RESUMO

Since the discovery of carbon nanotubes (CNTs), their remarkable properties make them ideal candidates to reinforce in advanced composites. In this attempt, an enhancement of mechanical properties of high density polyethylene (HDPE) by adding 1 wt% of CNTs is studied using Dynamic mechanical and Thermal analyzer (DMTA). The chemically treated and functionalized CNTs were homogeneously dispersed with HDPE and the test samples were made using injection molding machine. Using DMTA, storage modulus (E'), loss modulus (E") and damping factor (tan delta) of the sample under oscillating load were studied as a function of frequency of oscillation and temperatures. The storage modulus decreases with an increase of temperature and increases by adding CNTs in the composites where the reinforcing effect of CNT is confirmed. It is concluded that the large scale polymer relaxations in the composites are effectively restrained by the presence of CNTs and thus the mechanical properties of nanocomposites increase. The transition frequency of loss modulus is observed at 1 Hz. The loss modulus decreases with an increase of temperature at below 1 Hz but opposite trend was observed at above 1 Hz. The shift factor could be predicted from Williams-Landel-Ferry (WLF) model which has good agreement with experimental results.

19.
J Am Soc Mass Spectrom ; 19(11): 1590-9, 2008 Nov.
Artigo em Inglês | MEDLINE | ID: mdl-18657437

RESUMO

The gas-phase acidities of the six dimethylphenol isomers were determined experimentally, by using the kinetic method, and theoretically, through quantum chemistry calculations. The experimental values, relative to the gas-phase acidity of phenol, are (in kJ mol(-1)): -1.76+/-0.76 (2,3-Me2C6H3OH), 1.78+/-0.29 (2,4-Me2C6H3OH), 0.83+/-0.58 (2,5-Me2C6H3OH), -4.39+/-0.89 (2,6-Me2C6H3OH), 5.38+/-1.08 (3,4-Me2C6H3OH), and 1.88+/-0.08 (3,5-Me2C6H3OH). This trend was discussed by considering the substituent effects on the thermodynamic stabilities both of the parent phenols and the corresponding phenoxide ions. The above acidity data, the literature values for 2-, 3-, and 4-methylphenol, and the substituent effects analysis allowed to develop a simple empirical method to estimate the acidity of any methyl-substituted phenol.


Assuntos
Xilenos/química , Gases/química , Cinética , Espectrometria de Massas , Modelos Químicos , Teoria Quântica , Termodinâmica
20.
J Org Chem ; 73(16): 6213-23, 2008 Aug 15.
Artigo em Inglês | MEDLINE | ID: mdl-18651776

RESUMO

The C-H bond dissociation enthalpies (BDEs) for the five- and six-membered ring alkanes, alkenes, and dienes were investigated and discussed in terms of conventional strain energies (SEs). New determinations are reported for cyclopentane and cyclohexane by time-resolved photoacoustic calorimetry and quantum chemistry methods. The C-H BDEs for the alkenes yielding the alkyl radicals cyclopenten-4-yl and cyclohexen-4-yl and the alpha-C-H BDE in cyclopentene were also calculated. The s-homodesmotic model was used to determine SEs for both the parent molecules and the radicals. When the appropriate s-homodesmotic model is chosen, the obtained SEs are in good agreement with the ones derived from group additivity schemes. The different BDEs in the title molecules are explained by the calculated SEs in the parent molecules and their radicals: (1) BDEs leading to alkyl radicals are ca. 10 kJ mol (-1) lower in cyclopentane and cyclopentene than in cyclohexane and cyclohexene, due to a smaller eclipsing strain in the five-membered radicals relative to the parent molecules (six-membered hydrocarbons and their radicals are essentially strain free). (2) C-H BDEs in cyclopentene and cyclohexene leading to the allyl radicals are similar because cyclopenten-3-yl has almost as much strain as its parent molecule, due to a synperiplanar configuration. (3) The C-H BDE in 1,3-cyclopentadiene is 27 kJ mol (-1) higher than in 1,4-cyclohexadiene due to the stabilizing effect of the conjugated double bond in 1,3-cyclopentadiene and not to a destabilization of the cyclopentadienyl radical. The chemical insight afforded by group additivity methods in choosing the correct model for SE estimation is highlighted.


Assuntos
Cicloexanos/química , Ciclopentanos/química , Cicloexenos/química , Radicais Livres/química , Fotoquímica , Termodinâmica
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