7,652 research outputs found
Polymer crystal-melt interfaces and nucleation in polyethylene
Kinetic barriers cause polymers to crystallize incompletely, into nanoscale
lamellae interleaved with amorphous regions. As a result, crystalline polymers
are full of crystal-melt interfaces, which dominate their physical properties.
The longstanding theoretical challenge to understand these interfaces has new
relevance, because of accumulating evidence that polymer crystals often
nucleate via a metastable, partially ordered "rotator" phase. To test this idea
requires a theory of the bulk and interfacial free energies of the critical
nucleus. We present a new approach to the crystal-melt interface, which
represents the amorphous region as a grafted brush of loops in a
self-consistent pressure field. We combine this theory with estimates of bulk
free energy differences, to calculate nucleation barriers and rates via rotator
versus crystal nuclei for polyethylene. We find rotator-phase nucleation is
indeed favored throughout the temperature range where nucleation is observed.
Our methods can be extended to other polymers
Shape Fluctuations of a Droplet Containing a Polymer
We consider the problem of an ideal polymer confined in a droplet. When the
droplet radius is smaller than the (unconfined) polymer radius of gyration, the
polymer entropy will depend on the droplet shape. We compute the resulting
surface free energy. Using parameters appropriate for polymers confined in
microemulsions, we find that the polymer and bending surface energies are
comparable for the lowest modes. Finally, we argue that chain self-avoidance
will decrease the strength of the polymer contribution to the surface energy.Comment: Revtex, 12 pages, one figur
Impact of payments for environmental services and protected areas on local livelihoods and forest conservation in northern Cambodia
The potential impacts of payments for environmental services (PES) and protected areas (PAs) on environmental outcomes and local livelihoods in developing countries are contentious and have been widely debated. The available evidence is sparse, with few rigorous evaluations of the environmental and social impacts of PAs and particularly of PES. We measured the impacts on forests and human well-being of three different PES programs instituted within two PAs in northern Cambodia, using a panel of intervention villages and matched controls. Both PES and PAs delivered additional environmental outcomes relative to the counterfactual: reducing deforestation rates significantly relative to controls. PAs increased security of access to land and forest resources for local households, benefiting forest resource users but restricting households’ ability to expand and diversify their agriculture. The impacts of PES on household well-being were related to the magnitude of the payments provided. The two higher paying market-linked PES programs had significant positive impacts, whereas a lower paying program that targeted biodiversity protection had no detectable effect on livelihoods, despite its positive environmental outcomes. Households that signed up for the higher paying PES programs, however, typically needed more capital assets; hence, they were less poor and more food secure than other villagers. Therefore, whereas the impacts of PAs on household well-being were limited overall and varied between livelihood strategies, the PES programs had significant positive impacts on livelihoods for those that could afford to participate. Our results are consistent with theories that PES, when designed appropriately, can be a powerful new tool for delivering conservation goals whilst benefiting local people. El Impacto de los Pagos por Servicios Ambientales y Áreas Protegidas sobre la Subsistencia Local y la Conservación del Bosque en el Norte de Camboya RESUMEN: Los impactos potenciales de los pagos por servicios ambientales (PSA) y áreas protegidas (APs) sobre los resultados ambientales y las subsistencias locales en los países en desarrollo son polémicos y se han debatido ampliamente. La evidencia disponible es escasa; ha habido pocas evaluaciones rigurosas de los impactos ambientales y sociales de las APs y particularmente los PSA. Medimos el impacto sobre los bosques y el bienestar humano en tres diferentes programas de PSA que se llevan a cabo dentro de dos APs en el norte de Camboya usando un panel de aldeas de intervención y controles emparejados. Tanto los PSA como las APs brindaron resultados ambientales adicionales en relación a los contrafácticos, esto quiere decir que redujeron las tasas de deforestación significativamente en relación a los controles. Las áreas protegidas incrementaron el acceso seguro a los recursos del suelo y el bosque para las viviendas locales, beneficiando a los usuarios de los recursos del bosque pero restringiendo la habilidad de las viviendas para expandirse y diversificar su agricultura. Los impactos de los pagos por servicios ambientales sobre el bienestar de las viviendas estuvieron relacionados con la magnitud de los pagos proporcionados. Los dos programas de PSA de mayor paga y con conexión al mercado tuvieron impactos positivos significativos, mientras que un programa de menor paga con el objetivo de proteger a la biodiversidad no tuvo un efecto detectable sobre las viviendas, a pesar de sus resultados ambientales positivos. Las viviendas que se inscribieron a los programas de PSA con mayor paga, sin embargo, necesitaban típicamente más bienes capitales, por lo que eran menos pobres y tenían mayor seguridad alimentaria que otros aldeanos. Por esto, mientras los impactos de las APs sobre el bienestar de las viviendas fueron limitados en general y variaron dependiendo de las estrategias de subsistencia, los programas de PSA tuvieron impactos positivos significativos sobre las viviendas para aquellos que podían costear participar. Nuestros resultados son congruentes con las teorías de que los PSA, cuando se designan apropiadamente, pueden ser una herramienta poderosa y novedosa para obtener objetivos de conservación mientras se beneficia a la gente local
Impacts of Protected Areas on Local Livelihoods in Cambodia
SummaryImpact evaluation methods (mixed effects models and matching) were used to investigate the effect of protected areas (PAs) on poverty and livelihoods in Cambodia, comparing households inside PAs with bordering villages and controls. There was no evidence that PAs exacerbated local poverty or reduce agricultural harvests in comparison with controls. Households bordering the PAs were significantly better off due to greater access to markets and services. Non-timber forest product (NTFP) collectors inside PAs were significantly better off than controls and had greater rice harvests, because they had more secure access to land and forest resources. The PAs in Cambodia therefore have some positive impacts on households that use forest and land resources for their livelihoods
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