Use in polymer studies

Infrared (IR) spectroscopy has been widely used in polymer studies for the assignment of molecular structure and for monitoring changes in the arrangement of chemical bonds (1.8-11). 

The aim of this chapter is to introduce the reader to the application of positron annihilation techniques to polymers. An extensive review of the large volume of publications related to positron studies in polymers will not be presented. Rather it is intented to introduce the reader to the theory and techniques used in polymer studies and indicate the types of information that can be obtained about different polymer systems. The main focus of this chapter will be on the use of positron annihilation lifetime spectroscopy (PAL) in polymer studies. Chapter 11 discusses the use of monoenergetic slow positron beams used to study polymers surfaces. One of the interesting new developments in the application of positron annihilation techniques in polymers is the positron age-momentum correlation technique (AMOC). This technique promises to shed new light on the mechanisms of positronium formation and annihilation in polymer systems. A more detailed discussion of this technique can be found elswhere in this text. 

ESR spectroscopy was mainly developed to study the interaction energies of a parmagnetic atom in a constant magnetic field. It has therefore been mainly used to characterise rather than identify paramagnetic species. ESR has been used in polymer studies to identify the radical species generated by various processes, to determine the number of radicals produced and to follow the decay of radicals with time and environment. 

Isothermal calorimeters have also been used in polymer studies and primarily in investigations of heats and rates of polymerization reactions 28—37). Shielding and controls in reaction calorimeters are not as critical as in nonisothermal calorimeters used in spedfic heat measurement since most polymerization reactions are accompanied by relatively large enthalpy changes. On the other hand special attention must be jaid in the selection of the calorimeter material for a particular combination of monomers, solvents, catalysts, etc, to ensure inertness of the reactor wall to reaction components. Frisch and Mackle describe an aneroid high precision, semi-micro reaction calorimeter (Fig. 2) usable for a wide range of reaction systems including those in which one component is a gas. 

Thermomechanical Analysis. The equipment used in thermomechanical analysis (TMA) is similar in principle to that for TD, but provision is made for applying various types of load to the specimen, so that penetration, extension, and flexure can be measured. This approach to analyzing such modes of deformation is illustrated schematically in Figure 15. The technique finds most use in polymer studies, as in the determination of glass-transition and softening temperatures for thin films and shrinkage characteristics of fibers. 

The use of physical methods assumes that the theories involved are completely sound and the results obtained by their use quite unambiguous. This is hardly the case with modulus-cross link density, or with the swelling-cross link density relationships both widely used in polymer studies. 

The development of modem FTIR spectrometers has been accompanied by a rapid evolution of many novel sampling techniques [31]. These include making routinely practicable some long-established methods such as diffuse reflectance, microscopy and specular reflectance, and the introduction of new procedures such as photoacoustic (PA) measurements. All of these have found extensive uses in polymer studies and characterisations [32]. For instance. 

The large variety of techniques available make it possible to do many different kinds of experiments. There are many others which I have not discussed. For example, light scattering has been shown to be very useful in polymer studies (Beck etal, 1977). Figure 8 gives the time resolution for each of the techniques discussed here. 

The molecules used in the study described in Fig. 2.15 were model compounds characterized by a high degree of uniformity. When branching is encountered, it is generally in a far less uniform way. As a matter of fact, traces of impurities or random chain transfer during polymer preparation may result in a small amount of unsuspected branching in samples of ostensibly linear molecules. Such adventitious branched molecules can have an effect on viscosity which far exceeds their numerical abundance. It is quite possible that anomalous experimental results may be due to such effects. 

Applications of ISS to polymer analysis can provide some extremely useful and unique information that cannot be obtained by other means. This makes it extremely complementary to use ISS with other techniques, such as XPS and static SIMS. Some particularly important applications include the analysis of oxidation or degradation of polymers, adhesive failures, delaminations, silicone contamination, discolorations, and contamination by both organic or inorganic materials within the very outer layers of a sample. XPS and static SIMS are extremely comple-mentar when used in these studies, although these contaminants often are undetected by XPS and too complex because of interferences in SIMS. The concentration, and especially the thickness, of these thin surfiice layers has been found to have profound affects on adhesion. Besides problems in adhesion, ISS has proven very useful in studies related to printing operations, which are extremely sensitive to surface chemistry in the very outer layers. 

The main experimental techniques used to study the failure processes at the scale of a chain have involved the use of deuterated polymers, particularly copolymers, at the interface and the measurement of the amounts of the deuterated copolymers at each of the fracture surfaces. The presence and quantity of the deuterated copolymer has typically been measured using forward recoil ion scattering (FRES) or secondary ion mass spectroscopy (SIMS). The technique was originally used in a study of the effects of placing polystyrene-polymethyl methacrylate (PS-PMMA) block copolymers of total molecular weight of 200,000 Da at an interface between polyphenylene ether (PPE or PPO) and PMMA copolymers [1]. The PS block is miscible in the PPE. The use of copolymers where just the PS block was deuterated and copolymers where just the PMMA block was deuterated showed that, when the interface was fractured, the copolymer molecules all broke close to their junction points The basic idea of this technique is shown in Fig, I. 

The simplest model of polymers comprises random and self-avoiding walks on lattices [11,45,46]. These models are used in analytical studies [2,4], in particular in the numerical implementation of the self-consistent field theory [4] and in studies of adsorption of polymers [35,47-50] and melts confined between walls [24,51,52]. 

Commercial grades of PVP, K-15, K-30, K-90, and K-120 and the quaternized copolymer of vinylpyrrolidone and dimthylaminoethylmethacrylate (poly-VP/ DMAEMA) made by International Specialty Products (ISP) were used in this study. PEO standard calibration kits were purchased from Polymer Laboratories Ltd. (PL), American Polymer Standards Corporation (APSC), Polymer Standards Service (PSS), and Tosoh Corporation (TSK). In addition, two narrow NIST standards, 1923 and 1924, were used to evaluate commercial PEO standards. Deionized, filtered water, and high-performance liquid chromatography grade methanol purchased from Aldrich or Fischer Scientific were used in this study. Lithium nitrate (LiN03) from Aldrich was the salt added to the mobile phases to control for polyelectrolyte effects. 

After a temptative structure-based classification of different kinds of polymorphism, a description of possible crystallization and interconversion conditions is presented. The influence on the polymorphic behavior of comonomeric units and of a second polymeric component in miscible blends is described for some polymer systems. It is also shown that other characterization techniques, besides diffraction techniques, can be useful in the study of polymorphism in polymers. Finally, some effects of polymorphism on the properties of polymeric materials are discussed. 

The techniques used in the work have generally been spectroscopic visible-uv for quantitative determinations of species concentrations and infrared-Raman for structural aspects of the polymer. Although the former has often been used in the study of plutonium systems, there has been considerably less usage made of the latter in the actinide hydrolysis mechanisms. 

A different approach, although stdl working with essentially non-fiinctional polymers has been exemplified [114,115], in which, a 100% solid (solvent free) hot melt has been irradiated to produce pressure-sensitive adhesives with substantially improved adhesive properties. Acrylic polymers, vinyl acetate copolymers with small amounts of A,A -dimethylaminoethyl methacrylate, diacetone acrylamide, A-vinyl pyrrohdone (NVP) or A A have been used in this study. Polyfunctional acrylates, such as trimethylolpropane trimethacrylate (TMPTMA) and thermal stabilizers can also be used. 

FIGURE 6 Molecular structures of poIy(CTTE), poly(CTTH), and poly(CTTP), a homologous series of tyrosine-derived polymers used in a study of the effect of the C-terminus protecting group on the materials properties of the resulting polymers. Cbz" stands for the benzyloxycarbonyl group (47). 

We thank J. C. Rowell for the intrinsic viscosity data for the polymer samples used in this study. 

Several assumptions were made in using the broad MWD standard approach for calibration. With some justification a two parameter equation was used for calibration however the method did not correct or necessarily account for peak speading and viscosity effects. Also, a uniform chain structure was assumed whereas in reality the polymer may be a mixture of branched and linear chains. To accurately evaluate the MWD the polymer chain structure should be defined and hydrolysis effects must be totally eliminated. Work is currently underway in our laboratory to fractionate a low conversion polydlchlorophosphazene to obtain linear polymer standards. The standards will be used in polymer solution and structure studies and for SEC calibration. Finally, the universal calibration theory will be tested and then applied to estimate the extent of branching in other polydlchlorophosphazenes. 

Table 10.1 Characteristics of polymers used in this study, a indicates the label content.

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