Average customer rating:
|
Organic Electronic Materials : Conjugated Polymers and Low Molecular Weight Organic Solids (Springer Series in Materials Science) (Springer Series in Materials Science)
Manufacturer: Springer
ProductGroup: Book
Binding: Hardcover
General
| Electrical & Electronics
| Engineering
| Professional & Technical
| Subjects
| Books
General
| Optics
| Electrical & Electronics
| Engineering
| Professional & Technical
| Subjects
| Books
General
| Electronics
| Electrical & Electronics
| Engineering
| Professional & Technical
| Subjects
| Books
General
| Engineering
| Professional & Technical
| Subjects
| Books
General
| Materials Science
| Engineering
| Professional & Technical
| Subjects
| Books
Mechanical Properties of Solids
| Materials Science
| Engineering
| Professional & Technical
| Subjects
| Books
Polymer Science
| Materials Science
| Engineering
| Professional & Technical
| Subjects
| Books
Materials
| Chemical
| Engineering
| Professional & Technical
| Subjects
| Books
Polymer Chemistry
| Chemical
| Engineering
| Professional & Technical
| Subjects
| Books
Organic
| Chemistry
| Professional Science
| Professional & Technical
| Subjects
| Books
General
| Physics
| Professional Science
| Professional & Technical
| Subjects
| Books
General
| Science
| Subjects
| Books
General
| Physics
| Science
| Subjects
| Books
Chemical Physics
| Physics
| Science
| Subjects
| Books
Condensed Matter
| Solid-State Physics
| Physics
| Science
| Subjects
| Books
General
| Organic
| Chemistry
| Science
| Subjects
| Books
Polymers & Macromolecules
| Chemistry
| Science
| Subjects
| Books
Look Inside Science Books
| Trip
| Specialty Stores
| Books
All Amazon Upgrade
| Amazon Upgrade
| Stores
| Books
Engineering
| Amazon Upgrade
| Stores
| Books
Professional & Technical
| Amazon Upgrade
| Stores
| Books
Science
| Amazon Upgrade
| Stores
| Books
All Titles
| Qualifying Textbooks - Fall 2007
| Stores
| Books
Professional
| Qualifying Textbooks - Fall 2007
| Stores
| Books
Science
| Qualifying Textbooks - Fall 2007
| Stores
| Books
Accessories:
-
The Physics of Polymers: Concepts for Understanding Their Structures and Behavior
-
Hydrogen Bonded Polymers (Advances in Polymer Science)
-
Oligomers - Polymer Composites -Molecular Imprinting (Advances in Polymer Science)
ASIN: 3540667210 |
Product Description
Studies on the electronic properties of conjugated polymers and low molecular weight organic solids have been of increasing interest in recent years. This book is organized into two parts dedicated to these two classes of materials. For each part a general introductory review provides background knowledge of the language and of the main points required for understanding the book's contents. The reviews that follow provide a more complete understanding of the underlying physics of the materials through discussion of the interconnected topics. Theoretical concepts, models and methods are overviewed; this is used to support the explanation of the physical and chemical properties of these materials. The presentation of selected aspects of experimental research greatly contributes to the basic understanding of organic electronic materials.
Book Description
Provides an introduction to this exciting subject with chapters covering natural and synthetic polymers, colloids, surfactants, and liquid crystals, highlighting the many and varied applications of these materials.
- The only introductory book on this subject
- Cutting-edge introduction to a new and emerging subject area
- An interdisciplinary approach suitable of chemistry, materials science, physics, and biochemistry courses
- Copious end-of-chapter problems
Customer Reviews:
A concised and unified overview on soft matter.......2004-01-19
This book contains a concised overview on soft matter physics or what chemists use to call colloid chemistry. As stated by the author in the Preface, it has been written primarily for undergraduate students. However, it should be also useful to researchers that are looking for a unified view of the subject. The book is organized in five chapters: 1) Introduction, 2) Polymers, 3) Colloids, 4) Amphiphiles, and 5) Liquid crystals. Each chapter includes both FURTHER READING and QUESTIONS sections (numerical solutions to the questions are provided at the end of the book). All in all this quasi-pocket book appear to have reached its scope, I think, and hence I rated it with five stars. Readers that want to dig deeper into this subject may consult the book Soft Matter Physics: an Introduction, by Kleman and Lavrentovich (648 pp). Those wishing to take an intermediate step, may consult the recently published book Soft Condensed Matter by Jones (230 pp).
Average customer rating:
|
Organic Materials for Photonics: Science and Technology (European Materials Research Society Monographs, Vol 6)
Manufacturer: North-Holland
ProductGroup: Book
Binding: Hardcover
General
| Engineering
| Professional & Technical
| Subjects
| Books
General
| Materials Science
| Engineering
| Professional & Technical
| Subjects
| Books
Mechanical Properties of Solids
| Materials Science
| Engineering
| Professional & Technical
| Subjects
| Books
Polymer Science
| Materials Science
| Engineering
| Professional & Technical
| Subjects
| Books
Polymer Chemistry
| Chemical
| Engineering
| Professional & Technical
| Subjects
| Books
Physical
| Properties
| Materials
| Engineering
| Professional & Technical
| Subjects
| Books
General
| Science
| Subjects
| Books
General
| Physics
| Science
| Subjects
| Books
Optics
| Physics
| Science
| Subjects
| Books
Condensed Matter
| Solid-State Physics
| Physics
| Science
| Subjects
| Books
Optoelectronics
| Circuitry
| Computer Science
| Computers & Internet
| Subjects
| Books
ASIN: 0444899162 |
Average customer rating:
|
Metal Catalysed Reactions in Ionic Liquids (Catalysis by Metal Complexes)
Paul J. Dyson , and
Tilmann J. Geldbach
Manufacturer: Springer
ProductGroup: Book
Binding: Hardcover
General & Reference
| Chemistry
| Science
| Subjects
| Books
Inorganic
| Chemistry
| Science
| Subjects
| Books
General
| Organic
| Chemistry
| Science
| Subjects
| Books
General
| Physical & Theoretical
| Chemistry
| Science
| Subjects
| Books
General
| Science
| Subjects
| Books
Clinical Chemistry
| Pathology
| Specialties
| Medicine
| Subjects
| Books
Inorganic
| Chemistry
| Professional Science
| Professional & Technical
| Subjects
| Books
Organic
| Chemistry
| Professional Science
| Professional & Technical
| Subjects
| Books
Physical & Theoretical
| Chemistry
| Professional Science
| Professional & Technical
| Subjects
| Books
Clinical Chemistry
| Pathology
| Internal Medicine
| Medicine
| Medical
| Professional & Technical
| Subjects
| Books
Look Inside Science Books
| Trip
| Specialty Stores
| Books
All Amazon Upgrade
| Amazon Upgrade
| Stores
| Books
Medicine
| Amazon Upgrade
| Stores
| Books
Professional & Technical
| Amazon Upgrade
| Stores
| Books
Science
| Amazon Upgrade
| Stores
| Books
All Titles
| Qualifying Textbooks - Fall 2007
| Stores
| Books
Medicine
| Qualifying Textbooks - Fall 2007
| Stores
| Books
Professional
| Qualifying Textbooks - Fall 2007
| Stores
| Books
Science
| Qualifying Textbooks - Fall 2007
| Stores
| Books
ASIN: 140203914X |
Book Description
This book represents first non-edited book on the subject of metal catalysed reactions in ionic liquids and covers the literature from its origins until early 2005. Following a general introduction to the field of biphasic/mutliphasic catalysis, the synthesis, functionalisation and fundamental properties of ionic liquids relevant to catalysis are described. In the ensuing chapters catalysed reactions are discussed according to their type, encompassing hydrogenation, hydroformylation, oxidation, C-C coupling reactions, metathesis, dimerisation, polymerisation and more. Trends, generalisations, advantages and disadvantages of ionic liquids for specific reaction types are discussed. Specific processes such as supported ionic liquid phase catalysis, continuous processes using CO
2 extraction and nanoparticle catalysis are covered. The book should be of interest to all those working in catalysis/green chemistry, in particular, to advanced level undergraduate and graduate students and all those researching or contemplating research in bi- or multiphasic catalysis using ionic liquids.
Average customer rating:
|
Properties of Aluminum Gallium Arsenide (E M I S Datareviews Series)
Manufacturer: Institution of Electrical Engineers
ProductGroup: Book
Binding: Hardcover
Semiconductors
| Electrical & Electronics
| Engineering
| Professional & Technical
| Subjects
| Books
General
| Engineering
| Professional & Technical
| Subjects
| Books
General
| Materials Science
| Engineering
| Professional & Technical
| Subjects
| Books
Mechanical Properties of Solids
| Materials Science
| Engineering
| Professional & Technical
| Subjects
| Books
Reference
| Engineering
| Professional & Technical
| Subjects
| Books
General
| Science
| Subjects
| Books
General
| Physics
| Science
| Subjects
| Books
General
| Electromagnetism
| Physics
| Science
| Subjects
| Books
Condensed Matter
| Solid-State Physics
| Physics
| Science
| Subjects
| Books
Applied
| Physics
| Science
| Subjects
| Books
Engineering
| Reference
| Science
| Subjects
| Books
General
| Organic
| Chemistry
| Science
| Subjects
| Books
Look Inside Science Books
| Trip
| Specialty Stores
| Books
ASIN: 0852965583 |
Book Description
AlGaAs is the most widely studied and applied of the ternary semiconductors. An international array of 30 experts have contributed under the editorship of a world authority on AlGaAs, Dr. S. Adachi of Gunma University, Japan. Subjects covered: structural, mechanical, elastic, lattice vibrational and thermal properties; band structure; optical properties; electro-optical properties; carrier transport; surfaces, interfaces and contacts; impurity and defect centres; lattice dislocations, 2D carrier transport, real space transfer band offsets and electro-optic effects in AlGaAs/GaAs heterostructures.
Customer Reviews:
still relevant.......2005-01-29
Adachi's book is still quite useful, though published in 1993. He compiled a mass of published research on AlGaAs in the literature, and presented it here in a very useful form for you. He explains why AlGaAs is so interesting in heterostructure design. Because, in part, it can be used with layers of GaAs, with minimal dislocations, and compatible coefficients of thermal expansion. Vital considerations for practical chip design.
Maybe though, he should consider updating his book. There has been significant ongoing work on AlGaAs in the interim, and he might want to expand some sections.
Average customer rating:
- A comprehensive polymer physics book!
- A Decent, Instructive Book for Serious Students
|
The Physics of Polymers: Concepts for Understanding Their Structures and Behavior
Gert R. Strobl
Manufacturer: Springer
ProductGroup: Book
Binding: Paperback
General
| Materials Science
| Engineering
| Professional & Technical
| Subjects
| Books
Mechanical Properties of Solids
| Materials Science
| Engineering
| Professional & Technical
| Subjects
| Books
Polymer Science
| Materials Science
| Engineering
| Professional & Technical
| Subjects
| Books
Materials
| Chemical
| Engineering
| Professional & Technical
| Subjects
| Books
Polymer Chemistry
| Chemical
| Engineering
| Professional & Technical
| Subjects
| Books
Organic
| Chemistry
| Professional Science
| Professional & Technical
| Subjects
| Books
General
| Physics
| Professional Science
| Professional & Technical
| Subjects
| Books
Solid State Physics
| Physics
| Professional Science
| Professional & Technical
| Subjects
| Books
General
| Science
| Subjects
| Books
General
| Physics
| Science
| Subjects
| Books
General
| Solid-State Physics
| Physics
| Science
| Subjects
| Books
Condensed Matter
| Solid-State Physics
| Physics
| Science
| Subjects
| Books
Chemical Physics
| Physics
| Science
| Subjects
| Books
General
| Organic
| Chemistry
| Science
| Subjects
| Books
Look Inside Science Books
| Trip
| Specialty Stores
| Books
All Amazon Upgrade
| Amazon Upgrade
| Stores
| Books
Engineering
| Amazon Upgrade
| Stores
| Books
Professional & Technical
| Amazon Upgrade
| Stores
| Books
Science
| Amazon Upgrade
| Stores
| Books
All Titles
| Qualifying Textbooks - Fall 2007
| Stores
| Books
Professional
| Qualifying Textbooks - Fall 2007
| Stores
| Books
Science
| Qualifying Textbooks - Fall 2007
| Stores
| Books
Similar Items:
-
Polymer Physics (Chemistry)
-
An Introduction to the Mechanical Properties of Solid Polymers
-
The Theory of Polymer Dynamics (International Series of Monographs on Physics)
-
Principles of Polymer Chemistry (George Fisher Baker Non-Resident Lec)
-
Introduction to Physical Polymer Science
ASIN: 3540632034 |
Book Description
Polymer Physics is one of the key lectures not only in polymer science but also in materials science. Strobl presents in his textbook the elements of polymer physics to the necessary extent in a very didactical way. His main focus lays on the concepts of polymer physics, not on theoretical aspects or mere physical methods. He has written the book in a personal style evaluating the concepts he is dealing with. Every student in polymer and materials science will be happy to have it on his shelf.
Customer Reviews:
A comprehensive polymer physics book!.......2003-01-02
This is a comprehensive polymer physics book, each chapter is well written with adequate depth of coverage. Most definitely the book one must pick to delve into dynamics, thermodynamics, scattering and crystallization, and get to the level of appreciating the complexity and beauty of current research and understanding in the field of polymer physics. Highly recommended!
A Decent, Instructive Book for Serious Students.......2001-05-26
I'm no expert on polymers, but I've been exposed to a little on the subject through my work and classes, so I picked this up on sale. The beginnings of the chapters seem informative for one who wants a soundbite rather than the full meaty treatment. On a handful of occasions I've found a few numbers and figures from the book useful.
However, this is not a book for a beginner who wants to gain a little familiarity of the subject without serious study (not like, say, Saleh and Teich's optics book, where one can pick a topic and learn a little about it). When he decides to delve into theory the derivations can only be understood by sitting down and going through them by hand. It's not that steps are skipped or any other such atrocious practice, but simply that this is a book for people working hard to understand and join the field, not outsiders who want to gain some familiarity with it.
So, I'd say it's meaty but understandable for beginners who are serious about studying polymer physics, but not for people like me who encounter polymers from time to time and would like to gain a passing familiarity with the field. Also, it seems like a good reference even for experts, with the wealth of figures and charts and numbers and derivations.
Average customer rating:
|
Pulsed Electrochemical Detection in High-Performance Liquid Chromatography
William R. LaCourse
Manufacturer: Wiley-Interscience
ProductGroup: Book
Binding: Hardcover
Biochemistry
| Biological Sciences
| Science
| Subjects
| Books
Biotechnology
| Biological Sciences
| Science
| Subjects
| Books
Analytic
| Chemistry
| Science
| Subjects
| Books
Crystallography
| Chemistry
| Science
| Subjects
| Books
General & Reference
| Chemistry
| Science
| Subjects
| Books
Industrial & Technical
| Chemistry
| Science
| Subjects
| Books
Chromatography
| Chemistry
| Science
| Subjects
| Books
General
| Organic
| Chemistry
| Science
| Subjects
| Books
Electrochemistry
| Physical & Theoretical
| Chemistry
| Science
| Subjects
| Books
General
| Science
| Subjects
| Books
Crystallography
| Earth Sciences
| Science
| Subjects
| Books
Medical Technology
| Allied Health Professions
| Medicine
| Subjects
| Books
Clinical Chemistry
| Pathology
| Specialties
| Medicine
| Subjects
| Books
Biotechnology
| Bioengineering
| Engineering
| Professional & Technical
| Subjects
| Books
Analytic
| Chemistry
| Professional Science
| Professional & Technical
| Subjects
| Books
General & Reference
| Chemistry
| Professional Science
| Professional & Technical
| Subjects
| Books
Organic
| Chemistry
| Professional Science
| Professional & Technical
| Subjects
| Books
Medical Technology
| Allied Health Professions
| Medical
| Professional & Technical
| Subjects
| Books
Clinical Chemistry
| Pathology
| Internal Medicine
| Medicine
| Medical
| Professional & Technical
| Subjects
| Books
Look Inside Science Books
| Trip
| Specialty Stores
| Books
All Titles
| Qualifying Textbooks - Fall 2007
| Stores
| Books
ASIN: 0471119148 |
Book Description
A reliable, comprehensive, relevant view of HPLC and its applications
The development of HPLC-PED represents the successful marriage of two powerful analytical technologies and has resulted in the best technique for sensitive and direct detection of biological compounds with poor optical detection properties. PED has been used extensively for the determination of carbohydrates and other polar aliphatic compounds, and as a result, numerous methods have been developed to enable the analysis of a wide variety of samples. Over the years, many articles, anecdotal information, and misinformation have permeated the scientific community, with the possible consequence of confusion or uncertainty with regard to PED on the part of the analyst. Pulsed Electrochemical Detection in High-Performance Liquid Chromatography presents a reliable, comprehensive, and relevant review of HPLC-PED and its applications.
The book is divided into three major parts: background material necessary for a more thorough understanding of the principles and relevance of PED; an in-depth discussion of PED using voltammetry and other electroanalytical techniques and presenting the advantages, applicability, and optimization of all existing PED waveforms; and practical aspects of HPLC-PED, including a summary of the major applications and a look at future developments in the technique. Appendices include a pulsed voltammetry (PV) program specifically written to optimize pulsed amperometric detection (PAD) waveforms and all the known applications, categorized and listed in tabular form.
For analytical chemists; biochemists; carbohydrate chemists; biotechnologists; undergraduate, graduate, and postdoctoral students; and lab technicians working in a range of areas including the pharmaceutical, medical, and food and beverage industries, this eminently readable guide is the first reliable book-length treatment of how to use PED coupled with HPLC.
Book Description
This text provides explanations and examples of the primary chromatographic techniques for biomolecules (reversed-phase, size exclusion, ion-exchange, hydrophobic interaction and affinity). It also offers practical suggestions for their implementation and optimization. An extensive bibliography furnishes a wealth of background information.
Customer Reviews:
So brilliant that I cried--twice!.......2003-04-25
This is without a doubt the most brilliant and poignant depiction of HPLC and CE of biomolecules in modern history. My extreme hyperactivity disorder usually means that I'm not able to sit still for such weighty topics--not without large amounts of abused Ritalin; but Cunico's work was so compelling and finely wrought that I found myself spellbound, even when drunk, even when driving in the backseat of a stolen car across the Mexican border at Tecate. I am not going to recommend drinking heavily and stealing cars. But I do recommend BASIC HPLC and CE OF BIOMOLECULES by Robert L. Cunico. By the way, I am extremely drunk right now.
Basic HPLC and CE of Biomolecules.......2000-12-24
This is a book that should be on the desk of anyone who is involved in analytical development of biomolecules and biopharmaceuticals. The content is well organized and written in a format that is easy to grasp and understand. The content covers almost all the different modalities of HPLC analyses and applications and provides real life examples of the techniques. It includes innumerable suggestions on how best to develop a validatable HPLC method and analyze the data appropriately.
I would highly recommend this book to both new and old members of biotechnology, especially the developmental, analytical, and quality assurance groups where there is frequently a need to implement a solid reliable method for analyzing biomolecules in a more rational fashion.
Average customer rating:
|
High-Performance Liquid Chromatography and Lipids: A Practical Guide
William W. Christie
Manufacturer: Pergamon
ProductGroup: Book
Binding: Hardcover
General
| Science
| Subjects
| Books
Chromatography
| Chemistry
| Science
| Subjects
| Books
General
| Organic
| Chemistry
| Science
| Subjects
| Books
Physiology
| Basic Science
| Medicine
| Subjects
| Books
Organic
| Chemistry
| Professional Science
| Professional & Technical
| Subjects
| Books
ASIN: 0080342124 |
Average customer rating:
- This book explains things.
|
Giant Molecules: Here, There, and Everywhere...
Alexander Grosberg , and
Alexei R. Khokhlov
Manufacturer: Academic Press
ProductGroup: Book
Binding: Hardcover
General
| Biology
| Biological Sciences
| Science
| Subjects
| Books
Molecular Biology
| Biology
| Biological Sciences
| Science
| Subjects
| Books
General & Reference
| Chemistry
| Science
| Subjects
| Books
General
| Organic
| Chemistry
| Science
| Subjects
| Books
Polymers & Macromolecules
| Chemistry
| Science
| Subjects
| Books
General
| Science
| Subjects
| Books
Condensed Matter
| Solid-State Physics
| Physics
| Science
| Subjects
| Books
Clinical Chemistry
| Pathology
| Specialties
| Medicine
| Subjects
| Books
Polymer Science
| Materials Science
| Engineering
| Professional & Technical
| Subjects
| Books
Polymer Chemistry
| Chemical
| Engineering
| Professional & Technical
| Subjects
| Books
General
| Biology
| Biological Sciences
| Professional Science
| Professional & Technical
| Subjects
| Books
Molecular Biology
| Biology
| Biological Sciences
| Professional Science
| Professional & Technical
| Subjects
| Books
General & Reference
| Chemistry
| Professional Science
| Professional & Technical
| Subjects
| Books
Organic
| Chemistry
| Professional Science
| Professional & Technical
| Subjects
| Books
Clinical Chemistry
| Pathology
| Internal Medicine
| Medicine
| Medical
| Professional & Technical
| Subjects
| Books
General
| Chemistry
| Sciences
| New & Used Textbooks
| Stores
| Books
Organic & Inorganic
| Chemistry
| Sciences
| New & Used Textbooks
| Stores
| Books
ASIN: 0123041309 |
Book Description
Lively, informative, and easy-to-understand, this book is a fascinating read for scientists across many disciplines and the educated/interested layperson. Beautifully presented with eight pages of color illustrations,
Giant Molecules: Here, There, and Everywhere addresses the nature and role of giant molecules, which are the"source"for polymer chains. Grosberg and Khokhlov explain what giant molecules are, how they are formed, where they are applied, and whythey are important in everyday life and in science. The simple, non-technical style is complemented by full-color illustrations.
A CD-ROM enclosed with the book provides simulation of typical behavior of polymers, which is an extremely useful tool for teaching purposes. This feature will be of special interest to the expert in polymers across many disciplines, including molecularbiology, materials science, physics, and chemistry. The CD-ROM also allows the user to create a broad variety of movies simulating an almost arbitrary polymer system, customized to the user's specific needs.
Giant Molecules presents the subject matter much more clearly than the standard scientific text. It provides a unique opportunity to grasp the most advanced methods of polymer physics with the guidance of world leaders in the field. The book features a foreword written by Pierre-Gilles de Gennes, a Nobel Prize Laureate, and each chapter in this fascinating volume is preceded by a quote from such notable sources as Goethe, St. Exupery, and Mark Twain. Written in a simple, non-scientific style,
Giant Molecules should be understandable to interested readers at any level, and the color illustrations and CD-ROM add to the appeal of this lively book.
Key Features
* Includes a foreword written by Pierre-Gilles de Gennes, Nobel Laureate
* Written at a level both the scientist and non-scientist can appreciate
* Uses a clear, comprehensive manner to describe giant molecules
* Features a CD-ROM with movies showing the simulation of polymer behavior
* Presents a simple approach to the most complex phenomena, using very limited mathematics
* Considers aspects of polymers as materials
Customer Reviews:
This book explains things........2005-06-08
I am a physics professor working on semiconductor materials. Polymer is not my area, but they always attracts me a lot.
I can't believe this great book hasn't been reviewed yet. Yes, it is written by two Russian scientists. But who said Russians can only write rigorous math books? This book is not a monograph but for everyone interested in polymer physics, material or popular science. It explains a lot of phenomena in a clear, consise and humours language. There is a little math, not hard at all. Freshmen level calculus would be sufficient to understand the book. In fact, all through the book, the authors were showing the interactions between different factors. These factors are the reason that polymers have so many tantalizing and sometimes contradicting properties. To explain those, math is necessary.
The movies included in the accompanied CD are great, think about how much work is put on making them!
Books:
- Pattern Recognition and Machine Learning (Information Science and Statistics)
- Physical Chemistry
- Physical Metallurgy Principles (The Pws-Kent Series in Engineering)
- Plastic Part Design for Injection Molding : An Introduction (Spe Books.)
- Plutonium: A History of the World's Most Dangerous Element
- Practical HPLC Method Development, 2nd Edition
- Practical HPLC Method Development, 2nd Edition
- Protein NMR Spectroscopy: Principles and Practice
- Protein NMR Spectroscopy: Principles and Practice
- Quantitative Chemical Analysis
Books Index
Books Home
Recommended Books
- On Combat
- Love and Logic Magic for Early Childhood: Practical Parenting from Birth to Six Years
- Desperate Characters: A Novel
- Equine Massage: A Practical Guide
- How to Draw Action Dragonball Z
- History: Fiction or Science
- History: Fiction or Science
- Student's Solutions Manual Calculus: Graphical, Numerical, Algebraic
- History of Friedrich II of Prussia, Volume 21 and appendix
- General Hieu, Arvn: A Hidden Military Gem