Molecular Weight

5 Key excerpts on "Molecular Weight"

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  eBook - ePub

  Polymer Science and Innovative Applications

  Materials, Techniques, and Future Developments

  • Mariam Al Ali AlMaadeed, Deepalekshmi Ponnamma, Marcelo A. Carignano(Authors)
  • 2020(Publication Date)
  • Elsevier

    (Publisher)

  As polymer chains are of different lengths, the Molecular Weight of a polymer is described as an average Molecular Weight [23]. The Molecular Weight is an important characteristic of a polymer sample as several properties, for example, mechanical (stress–strain, impact, fatigue, creep, etc.) and thermal properties are influenced mainly by Molecular Weight and Molecular Weight distribution [24]. Electrical properties such as conductivity, dielectric constant, and dielectric loss as well as physical properties including viscosity, osmotic pressure, elevation in boiling point, depression in freezing point, etc., also depend on the Molecular Weight significantly [25 – 28]. The different types of average Molecular Weights are number average Molecular Weight (M n), weight average Molecular Weight (M w), viscosity average Molecular Weight (M v), and Z-average Molecular Weight (M z). The Molecular Weight of a polymer can be determined using a variety of methods, and it is possible to classify these methods into absolute, equivalent, and relative methods [23]. Absolute methods include the estimation of the Molecular Weight of a polymer directly from measured quantities without taking into account the physical/chemical properties of a polymer sample. However, knowing the chemical structure of the polymer sample is an additional requirement for the equivalent method of analysis. Relative methods are based on the measurement of properties that are a function of the physical properties as well as the chemical structure of polymers; wherein the Molecular Weight of polymer standards having similar physical properties and chemical structures as that of the polymer to be characterized are required

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  eBook - ePub

  Chemistry

  Concepts and Problems, A Self-Teaching Guide

  • Richard Post, Chad Snyder, Clifford C. Houk(Authors)
  • 2020(Publication Date)
  • Jossey-Bass

    (Publisher)

  4 Molecular and Formula Weights

  Since every molecule is made up of a definite and invariable number of component atoms, it follows that each kind of molecule has a definite and characteristic weight. You may ask, “How did anyone ever determine how many atoms of each kind are in a molecule of any pure substance?” We try to answer that question in this chapter. We may determine the Molecular Weight of a substance in a number of ways using instrumental techniques that are available today. Specific instrumental methods are not discussed here. Instead, we will rely upon the data that we get by analyzing substances and what you have learned so far in Chapters 1 through 3. We will depend upon the periodic table and the information it provides.

  This chapter will teach you how to determine the Molecular Weight and percentage composition of a compound when given its molecular formula. You will also be able to reverse the process to determine the molecular formula of a compound given the percentage composition of the compound (or the weight of each element per molecule) and its Molecular Weight. The important thing to remember is that every molecule consists of a definite number of atoms in a fixed ratio expressed as small whole numbers.

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  eBook - ePub

  Introduction to Physical Polymer Science

  • Leslie H. Sperling(Author)
  • 2015(Publication Date)
  • Wiley-Interscience

    (Publisher)

  Polymer Laboratories GPC and HPLC instrumentation by themselves provide relative Molecular Weights and Molecular Weight distributions, requiring calibration. However, once set up, Molecular Weights can be determined on a variety of polymers, at a rate of two determinations an hour or better. The addition of light-scattering provides absolute Molecular Weights.

  The Physical Electronics TRIFT II MS® provides absolute Molecular Weights at the low end of the polymer Molecular Weight range. It can also help determine end groups and other moieties.

  The Waters/Micromass instrument determines absolute Molecular Weights and distributions. This instrumentation is not ideal for very high Molecular Weights, but at lower Molecular Weights may provide the exact Molecular Weight for each species present.

  3.12 SOLUTION THERMODYNAMICS AND Molecular WeightS

  A knowledge of solution thermodynamics is critically important in determining the suitability of a solvent for a Molecular Weight determination. Once having decided on a suitable solvent, solution thermodynamics provides a basis for determining how an extrapolation to zero concentration is to be carried out.

  Below the Flory θ-temperature, polymer solutions may phase-separate. The higher the Molecular Weight is, the higher the upper critical solution temperature. At infinite Molecular Weight, the Flory θ-temperature is reached. Thus the Flory θ-temperature is defined by several different criteria:

  1. It is the temperature where A2 is zero for dilute solutions, and χ1 = 1/2.

  2. It is the temperature where the radius of gyration approximates that of the bulk polymer (see Chapter 5).

  3. It is the temperature at which an infinite Molecular Weight fraction would just precipitate (see Chapter 4).

  The Molecular Weight and polydispersity of polymers remain among the most important properties that are measured. The methods are divided into absolute methods, which determine the Molecular Weight from first principles, and relative methods, which depend on prior calibration. The latter are usually selected because they are fast and inexpensive. Values obtained from the several methods are summarized in Table 3.15

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  eBook - ePub

  Fundamentals of Polymer Engineering, Third Edition

  • Anil Kumar, Rakesh K. Gupta(Authors)
  • 2018(Publication Date)
  • CRC Press

    (Publisher)

  8     

  Measurement of Molecular Weight and Its Distribution

  The flow, optical, and mechanical properties of a polymer depend on its Molecular Weight and Molecular Weight distribution. This chapter discusses the different methods by which one may measure the number-average Molecular Weight, as well as the weight-average Molecular Weight,; both these quantities have been defined in earlier chapters. While is best measured with the help of membrane osmometry, the preferred technique for the determination of is static light scattering, and this method also provides information on the size of the polymer molecules in solution. The ratio of to is an important parameter known as the polydispersity index, but it is only one measure of the width of the Molecular Weight distribution. The entire distribution and the different moments of the distribution can be measured using gel-permeation chromatography, also known as size-exclusion chromatography. These techniques are described in detail in this chapter along with the measurement of intrinsic viscosity, a commonly encountered quantity that is attractive due to its simplicity. It, however, yields a “viscosity-average” Molecular Weight that is intermediate between the number-average and weight-average Molecular Weights.

  8.1    INTRODUCTION

  A solid polymer is a mosaic of structures. For a crystallizable homopolymer, for example, we can vary the amount and nature of the crystallinity and the shape and size of the crystals. In addition, we can vary the orientation of the polymer chains in both the crystalline and amorphous phases. This variation can be brought about by changing either the material variables or process conditions. The former include the chemical structure, the Molecular Weight and its distribution, the extent of chain branching, and the bulkiness of the side groups. The latter include the temperature and the deformation rate. It is the interplay within this multitude of variables that leads to the physical structure visible in the finished product. This structure, in turn, determines the properties of the solid polymer. In this chapter, we examine the methods of measuring the polymer’s Molecular Weight and its distribution. These quantities were defined in Chapter 1

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  eBook - ePub

  Polymer Characterization

  Physical Techniques, 2nd Edition

  • Dan Campbell, Richard A. Pethrick, Jim R. White(Authors)
  • 2017(Publication Date)
  • CRC Press

    (Publisher)

  2 Molecular mass determination

  2.1 Introduction

  The essential distinguishing characteristic of polymeric materials is their molecular size. The properties which have enabled polymers to be used in a diversity of applications derive almost entirely from their long chain macro-molecular nature. The molecular size of a monodisperse molecular substance is expressed in terms of its relative molar mass (RMM) (or molecular mass) which is the mass in grams of 1 mole of the substance relative to the mass of 1 mole (12 g) of 12 C. 1 mole of a substance contains Avogadro’s number (6.02205 × 1023 mol–1 ) of units (atoms, ions or molecules). Actual molar masses are given by the RMM multiplied by the atomic mass unit (amu) where 1 amu = 1.6606 × 10–27 kg. Thus in practice the RMM is expressed in units of g mol–1 or amu, although the units are normally omitted. The relative molar masses of polymers are related to the degree of polymerization X (the number of repeat units in the chain) and the relative molecular mass of the repeat unit,

  Mr

  by M =

  X Mr

  Non-network polymers, thermoplastics, uncured elastomers and thermosetting resins, consist of an assembly of molecules having a distribution of molecular sizes, i.e. they are poly disperse. In order to characterize fully these materials it is essential to have some means of defining and determining their molar masses and molar mass distributions. For crosslinked network structures the molecular size is considered to be essentially infinite so that the concept of molar mass is less useful and their properties are determined largely by the crosslink density.

  Condensation polymers will generally have a broader molecular mass distribution compared with vinyl monomers. The latter are easily obtained in a wide variety of relative molecular masses varying from 103 –107 amu whereas condensation polymers usually are obtained with values of 105

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