Today's children and adolescents are immersed in both traditional and new forms of digital media. Research on traditional media, such as television, has identifi ed health concerns and negative outcomes that correlate with the duration and content of viewing. Over the past decade, the use of digital media, including interactive and social media, has grown, and research evidence suggests that these newer media offer both benefi ts and risks to the health of children and teenagers. Evidence-based benefi ts identifi ed from the use of digital and social media include early learning, exposure to new ideas and knowledge, increased opportunities for social contact and support, and new opportunities to access health promotion messages and information. Risks of such media include negative health effects on sleep, attention, and learning; a higher incidence of obesity and depression; exposure to inaccurate, inappropriate, or unsafe content and contacts; and compromised privacy and confi dentiality. This technical report reviews the literature regarding these opportunities and risks, framed around clinical questions, for children from birth to adulthood. To promote health and wellness in children and adolescents, it is important to maintain adequate physical activity, healthy nutrition, good sleep hygiene, and a nurturing social environment. A healthy Family Media Use Plan (www. healthychildren. org/ MediaUsePlan) that is individualized for a specifi c child, teenager, or family can identify an appropriate balance between screen time/online time and other activities, set boundaries for accessing content, guide displays of personal information, encourage age-appropriate critical thinking and digital literacy, and support open family communication and implementation of consistent rules about media use.
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Infants, toddlers, and preschoolers are now growing up in environments saturated with a variety of traditional and new technologies, which they are adopting at increasing rates. Although there has been much hope for the educational potential of interactive media for young children, accompanied by fears about their overuse during this crucial period of rapid brain development, research in this area still remains limited. This policy statement reviews the existing literature on television, videos, and mobile/interactive technologies; their potential for educational benefit; and related health concerns for young children (0 to 5 years of age). The statement also highlights areas in which pediatric providers can offer specific guidance to families in managing their young children’s media use, not only in terms of content or time limits, but also emphasizing the importance of parent–child shared media use and allowing the child time to take part in other developmentally healthy activities.
In this paper, we examine the melt rheology of well-defined, model polymers where the long chain branching (LCB) is precisely known from the synthesis. All of these are made by the hydrogenation of polybutadiene, but they vary greatly in the level and type of LCB present. We find that all polymers that have LCB show a greater degree of shear thinning than linear chains. This applies both to those with a single branch (stars) and also to those with multiple branches per chain (such as combs). However, only molecules with multiple branches induce extensional thickening in a sample. Only a small amount of these comblike molecules, on the order of 5%, are needed to show this effect. We also show here how a new method of treating the shear data, the so-called Van Gurp−Palmen analysis, can give a more easily interpreted form of the results that can reveal the length and amount of branches in a sample. The insights generated from this work show the importance of access to well-defined polymers with several kinds of branching architecture for the development of a deeper understanding of polymer rheology.
We describe the synthesis and characterization of a number of polymers with well-defined structures that serve as models for polyethylene with long chain branching. All of them have been made by using anionic polymerization techniques and controlled chlorosilane chemistry to give nearly monodisperse polybutadienes with precise control of the number, length, and placement of long (M h w > 1500 g/mol) branches on each chain. This was followed by hydrogenation to give saturated polymers with the same well-defined long chain branching and the local structure of a typical linear low-density polyethylene. That is, both the backbones and the long branches had 17-25 ethyl branches per 1000 total carbons. Among the structures made were some with no long branches ("linears"), some with a single long branch ("stars"), others with exactly two branch points (the R-ω type, "H's", "super-H's", and "pom-poms"), and some with several long branches randomly distributed along the backbone ("combs"). Essentially all types of branching from a linear backbone can be made by the techniques described herein. While linear and symmetrical star models of polyethylene have been made previously, the other structures are the first examples of polyethylene models with multiple branches and precise control of the molecular architecture. We use the results given here to discuss how long chain branching can be detected in polyethylene. We also show how the branching structure controls chain dimensions. The Zimm-Stockmayer model works well to describe the sizes of the lightly branched molecules, but its predictions are too small for those with many long branches. This is presumably due to crowding of the branches. The rheological properties of these polymers will be described in subsequent publications.
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