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A simple, inexpensive method is described for the measurement of specific volumes of thermoplastic melts during extrusion at low shear stresses. Data for several polyolefins agree closely with specific volumes obtained by dilatometry or compression measurements on static melts. Data can be obtained at a variety of temperatures more rapidly than by dilatometry. The method described yields specific volumes essentially at atmospheric pressure, but extension to higher pressures and the measurement of compressibilities is possible, with more complicated apparatus. Simultaneous measurements can be also made of apparent shear rates, and these can be converted to absolute shear rates, and viscosities, by application of standard techniques. The simple procedure described does not permit the concurrent measurement of useful apparent shear and volume viscosities because the initial state of the system reflects operational as well as polymer properties. It is suggested that an extension of the present method would provide such data.

Two-dimensional time-domain 1H NMR was used to investigate annealed isotactic polypropylene in the solid phase. The spin–lattice relaxation in the laboratory frame and in the rotating frame were correlated with the shape of the free induction decay to identify and characterize relaxation components over the temperature range −120 to 120 °C. Several phase transitions were observed, and three distinct solid phases, with different chain mobilities, were detected. Two of these phases were identified as regions with different mobilities within the crystalline phase. The third phase was characterized by a high degree of isotropy in molecular motion. This phase, identified as the amorphous phase, appeared as the polymer was heated above a low-temperature (−45 °C) phase transition. All transitions observed at higher temperatures occurred exclusively in this phase. About one-third of the polymer chains reside between crystalline lamellae, whereas the majority form amorphous regions outside fibrils of multilamellar structure. Furthermore, the glass-to-rubber transition, occurring above −15 °C, consists of three stages. During the first stage, between −15 °C and 15 °C, regions with an increased segment mobility (labeled intermediate phase) appear gradually within the amorphous phase. At 15 °C, the intermediate phase consists of ∼10% of the polymer units, or one-third of the polymer units constituting the amorphous phase. Between 15 °C and 25 °C, the intermediate phase increases rapidly to 18%. This is associated with the appearance of semiliquid and liquid regions, likely within the intermediate phase. Polymer chain segments (and possibly entire chains) involved in the liquidlike phases exhibit heterogeneous molecular motion with a correlation frequency higher than 106 Hz. These two stages of glass-to-rubber transition occur within amorphous regions outside multilamellar structures. The third stage of the glass transition, appearing above 70 °C, is associated with the upper glass transition and occurs within the interlamellar amorphous phase. Finally, on a timescale of 100 ms or less, spin diffusion does not couple the amorphous regions outside fibrils with crystalline and amorphous regions within multilamellar fibrils. However, on a timescale of hundreds of milliseconds to seconds, all different regions within isotactic polypropylene are partially coupled. It is proposed that the relative magnitude of the crystalline magnetization, as observed in the T1ρ experiment, is a good measure of polymer crystallinity. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2487–2506, 2000

The intensities and lifetimes of positrons annihilating in low and high density poly(ethylene) (LDPE and HDPE) were measured. As expected the orthopositronium (oPs) pick-off lifetime and intensity increases with temperature; the relative values of the intensity are consistent with differential scanning calorimetry (DSC) results. Two other components with shorter lifetimes were deconvoluted from the complex exponential decay profiles. Detailed studies were carried out on quenched and annealed samples of LDPE and HDPE in an attempt to correlate the two shorter components with crystallization; both CsF and plastic scintillators were used. DSC analyses were also carried out. We conclude that the shortest lifetime component is caused by a combination of parapositronium (pPs) decay and the annihilation of free positrons in the crystalline regions and that the second longest lived component can also be associated with crystalline regions especially the crystalline/amorphous boundary.

The mechanical properties of blends of isotactic polypropylene and high-density polyethylene with a postconsumer resin (recycled dairy containers) were investigated over the entire composition range. Modification of these blends with an ethylene/propylene/diene copolymer or an ethylene/vinyl acetate copolymer was also investigated. Isotactic polypropylene/postconsumer resin blends have satisfactory impact and tensile properties at postconsumer resin contents of less than 50% for certain applications. At higher postconsumer resin contents, the tensile properties were adversely affected. The impact properties remained satisfactory. Addition of an ethylene/propylene/diene copolymer improved the mechanical properties of these blends to levels equal to or greater than those for neat isotactic polypropylene. Ethylene/vinyl acetate copolymers were also able to improve the mechanical properties, but not as efficiently as did the ethylene/propylene/diene copolymer. Blends of high-density polyethylene and a postconsumer resin had poor impact and tensile properties. Although the ethylene/propylene/diene copolymer and ethylene/vinyl acetate copolymers were able to improve these properties, the improvement was insufficient for general recycling, except at lower (≤25%) postconsumer resin contents. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 70: 2081–2095, 1998

In this report, we present the results of our investigation into the crystallization behaviour of polyproplene. The crystallization process was followed by hot-stage optical microscopy, differential scanning calorimetry, and dynamic mechanical methods. As well, nuclear magnetic resonance spin-spin relaxation techniques were employed to probe the morphology of the polypropylene. Nucleation in this isotactic polypropylene was athermal and appeared to be heterogeneous. The dependence of the onset temperature of crystallization on the annealing temperature and on the annealing time indicated that PP nuclei were able to survive the melting process. Nuclear magnetic resonance spin-spin relaxation experiments indicated that the PP melt contained a significant proportion of regions of high segment density. It was postulated that these ‘ordered’ regions acted as nucleation sites for PP crystallization, and that the number and size of these regions was determined by the annealing time and temperature. Addition of HDPE to PP resulted in melting-point depression and plasticization of the PP phase at lower HDPE contents. HDPE was able to penetrate the PP phase sufficiently at lower HDPE contents to reduce the number and size of regions of high segment density, thereby delaying the nucleation and subsequent crystallization of the PP phase.

With two-dimensional time-domain NMR analysis in the proton spin rotating frame, three phases are identifiable in the solid polyethylene. The major proton magnetization fraction is due to the polymer's crystalline region, where the motion is least isotropic and slowest. A magnetization fraction with intermediate relaxation rate is also intermediate in magnitude. This component is proposed to comprise chain loops on the surfaces of crystallites and effectively entangled chain segments. The most mobile fraction, which is most liquid-like with a T2 of near 1 ms at 120°C, is also the smallest. It is proposed that it is due to polymer chains in the amorphous phase. In the crystalline phase the chain motion is an unexpectedly effective relaxation mechanism at ∼50 KHz. This process, which involves propagation of a twisted region along the crystallite, as has been suggested before, is either not present in the other two phases of the HDPE or is less efficient because of the lower polymer density. The activation energies for the crystalline, intermediate and amorphous phases are estimated to be 34 ± 3, 13 ± 3, and 8 ± 2 Kcal/mol, respectively. The frequencies of the relaxation process at 120°C are 43 KHz, 350 KHz, and 1.5 MHz, respectively. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 2551–2558, 1997

The basic features of a three-step experimental process to produce supermicron polymer particles are described. First, a submicron emulsifier-free latex is prepared by a well-known technique. Second, the latex is aggregated by destabilizing with cetyl pyridinium chloride under constant stirring conditions, to yield roughly spherical clusters of 6-12 μ diameter. Third, the aggregates are stabilized with poly(vinyl alcohol) and internally coalesced by heating at or above the glass transition temperature. The final product particles have relatively smooth surfaces. Results are qualitatively interpreted in terms of a dynamic equilibrium where the aggregate size is determined by a balance between attractive interparticle potentials and stirring shear forces. Bimodal aggregate size distributions suggest the aggregate break-up mechanism may involve the erosion of individual latex particles and small fragments from the surface of aggregates. © 1996 John Wiley & Sons, Inc.

The mechanical properties of high-density polyethylene (HDPE)-rich i-PP/HDPE blends were studied. Two grades of HDPE were investigated, one with a melt viscosity close to that of the polypropylene (PP) and the other having a much lower melt viscosity. Compatibilization of the 10/90 i-PP/HDPE blend with three copolymers (an ethylene/propylene/diene [EPDM] copolymer and two ethylene/vinylacetate [EVA] copolymers, differing in their VA content) was also investigated. Blends of PP with the low melt viscosity HDPE displayed poor mechanical properties. It was not possible to improve these properties sufficiently with EPDM or EVA. In the case where viscosity matching was achieved between PP and HDPE, addition of i-PP (up to 30%) to HDPE resulted in a large drop in the impact strength of the blends, compared to that of the neat HDPE. A large drop (>50%) was also observed in the ultimate tensile elongation. However, the flexural modulus, yield stress, and ultimate tensile strength all increased with the introduction of i-PP into HDPE. Modification of these blends with an EPDM resulted in the return of all properties to values very close to those of the neat HDPE. The ultimate tensile elongation of the EPDM-modified i-PP/HDPE blend even exceeded that of the virgin HDPE. It was also found that although EVAs can be used to compatibilize these blends these additives were not as effective as was the EPDM. © 1996 John Wiley & Sons, Inc.

A low-density polyethylene was shear modified by repeated extrusions. These operations resulted in no detectable changes in the polymer molecular weight distribution. The apparent melt viscosity and elasticity decreased after extrusion, as is commonly observed in shear modification. These rheological effects were fully reversible by annealing the melt. Transverse proton relaxation measurements showed the existence of four identifiable different regions in the polymer melt. The range of relaxation times encompassed two orders of magnitude. These reflect the existence of regions with different degrees of conformational order. Shear modification results in the breakup of a minor proportion of the most constrained regions, which have the shortest relaxation times. The apparent “entanglements” that are inferred from rheological measurements probably reflect the coexistence of segments of more than one macromolecule in chain-folded domains, as indicated by molecular dynamics calculations. These results are evidence that the equilibrium state of LDPE is not a chain-entangled, highest entropic condition, but rather a lowest free energy state in which chain folding is favored. They are consistent with other longstanding observations, such as the failure of experiments to produce amorphous polyethylene and the apparent lack of mixing of HDPE and LDPE on a molecular level.

Monodisperse vinyl acetate/butyl acrylate (85/15) latexes were prepared with particle sizes between 200 and 1200 nm. These were used in a simple paint formulation with TiO 2 as the only pigment. Two optical methods were used for determination of the critical pigment volume concentration (CPVC) of these paints. These are specular gloss at an incidence angle of 85 ° and CIE Z (lightness) values of a blue tinted formulation. Both methods gave coincident results for the CPVC as a function of latex particle size. Our data fit the expression X = 1.44 Y 2.65 where X = number of latex particles/number of pigment particles, at CPVC and Y = diameter of pigment particles/diameter of latex particles. Earlier, Bowell reported the same exponential relation, with a different pre-exponential factor, for acrylic latexes. The common exponent is explained with a model in which the latex and pigment particles assume a condition close to minimum porosity during packing. CPVC reflects the packing of particles during the initial stages of dryness of the latex. The ‘universal’ relation reported here permits facile prediction of the effects of latex particle size on the CPVC and binding capacity in water-based paints.

Three commercially produced polymer samples (polyethylene, polypropylene, and polystyrene) have been analyzed in the melt state using proton nuclear magnetic resonance (NMR) T 2 relaxation methods using the Carr-Purcell-Meiboom-Gill (CPMG) spin-echo pulse sequence. Samples were run with exposure to air and again after extensive degassing at a vacuum of 10 -4 mmHg for periods of not less than 96 h. The comparison is made by initially considering the presence of microscopic voids in the samples as a source of local field inhomogeneity and how they affect the T 2 relaxation behavior. For PP and PS samples, degassing caused a decrease in all T 2 time constants associated with the multicomponent decays. The component intensities each of the time constants was also significantly altered. For the PE sample, degassing caused a decrease in the time constants associated with the amorphous material in the molten polymer. Examination of the fastest relaxing component of the three component decay showed approximate invariance in the T 2 decay constant. This result supports our previously reported model in which that fast relaxing component is attributable to regions of local order or high segmental density within the molten polyethylene, a remnant of the crystalline material which exists in the premelting bulk polymer. The results of this research are of particular significance to those who wish to use this NMR technique as a quantitative method of determination of NMR distinct morphological regions within heterogeneous polymers

Reported here is a novel technique, based on inverse gas chromatography (IGC), to measure the three-dimensional solubility parameters of nonionic surfactants—nonyl phenol ethoxylates—with different ethylene oxide chain lengths at two different temperatures. These solubility parameters can then be used to characterize the relative oil and water affinities of the surfactant. The three-dimensional solubility parameters of these surfactants were determined on the basis of the measured interaction parameters and known three-dimensional solubility parameters of the solvents (probe molecules) used in the IGC experiments. In particular, it was assumed, based on the regular solution theory, that the occurrence of a zero or negative interaction parameter is evidence of thermodynamic compatibility of the particular probe and surfactant. The three-dimensional solubility parameters of the surfactant are obtained by averaging the corresponding values of the compatible low-molecular-weight probe substances. In this way, combination of the lattice model of solutions (which allows negative enthalpies of solution) and the solubility parameter theory (which accounts for endothermic solutions only) is not required. As a result, measured negative interaction parameters are not a stumbling block for obtaining Hildebrand and three-dimensional solubility parameters. The method described is applicable to measurement of three-dimensional solubility parameters of all materials. Surfactants were used here as a convenient illustration only.

Past work has shown that addition of high-density polyethylene (HDPE) to polypropylene (PP) resulted in a significant reduction in the elongation at break and in the gate-region impact strength of injection molded articles. In this study, we report on the compatibilization of these blends with ethylene/propylene/diene monomer (EPDM) or ethylene/vinyl acetate (EVA) copolymers. EPDM is effective at improving the impact strength and gate puncture resistance of a 90/10 PP/HDPE blend. EVA, on the other hand, is effective at improving the impact strength and the elongation at break of this material. It was also found that EVA is a better impact modifier for PP than is EPDM. DSC analysis indicated that EPDM, when blended with PP, caused a significant reduction in the degree of crystallinity of the PP. This was not observed in the case of EVA/PP blends. Further, our results indicate that care must be taken in designating a particular material as a better compatibilizer than some other material. It is crucial that conditions be attached to the label compatibilizer, indicating in particular the desired mechanical properties of the blend. © 1996 John Wiley & Sons, Inc.

The amount of divinylbenzene (DVB) that can be incorporated in a one-shot dispersion copolymerization with styrene to form a coagulum-free latex is low (

An ethylene–octene linear low-density polyethylene (LLDPE) was treated with peroxide in a reactive extrusion system. A counterrotating nonintermeshing twin-screw extruder (System 2) was contrasted with a corotating intermeshing twin-screw machine (System 1). In System 2, the peroxide solution was pumped into the melted polymer, while it entered with the polymer pellets in the feed section of System 1. Molecular structure changes and the rheological behavior of peroxide-modified resins are similar in both operations but System 2 is much more effective. Much lower peroxide levels were needed in System 2. However, reactions in this setup were also more difficult to control. The presence of microgel was clearly evident in System 2 products but not in those made in System 1. The results of such reactive extrusion processes depend critically on the method of the peroxide feed and mixing conditions. Reaction conditions that favor optimum economy and peroxide efficiency are those which may compromise product homogeneity. © 1996 John Wiley & Sons, Inc.

The effects of reactive extrusion with peroxide have been compared for two linear low-density polyethylenes (LLDPEs). Resin C is a butene copolymer, while Resin B is an octene-ethylene copolymer. Both have similar molecular weight distributions, but Resin B is significantly richer in terminal vinyl groups. Under the same reactive extrusion conditions, Resin B is much more reactive. Resin C is easier to process in this system, producing products clean of microgel. Concurrently, however, a given peroxide level produces less molecular weight enhancement and long-branch formation in Resin C. Reactive extrusion with peroxide is suitable to effect improvements in the processability of LLDPEs for film applications, provided that the choice of peroxide and the process details are tailored to the characteristics of the particular LLDPE.

Matrix assisted laser desorption/ionisation time-of-flight mass spectrometry (MALDI-TOF MS) is used to mass characterise several low molecular weight, commercially available PS, PMMA and PEG samples. In addition, the utility of the technique for the study of polymer synthesis is demonstrated; different initiation and termination mechanisms leave dead chains with characteristic end groups which, in many cases, can be rapidly identified mass spectrometrically. There is broad agreement between MWDs derived from MALDI spectra and those obtained from conventional GPC analysis, but in some instances the differences are significant. Polydispersities obtained mass spectrometrically are invariably lower than those determined by GPC.

In emulsion polymerization processes it is important to control the surfactant level to avoid secondary nucleation (too much surfactant) and coagulation (too little). Persulfate initiators generate two types of surface active sulfate-containing species; anchored sulfate groups (10-20%) and mobile in situ surfactant (ISS; 50-70%). This paper examines the significance of the competition between these two species and an added surfactant, sodium dodecylbenzene sulfonate (SDBS). SDBS soap titrations were performed on five uncleaned methyl methacrylate/butyl acrylate latexes with particle sizes from 500 nm to 3.25 μm at volume fractions from 1 to 30%. Not surprisingly, traditional Langmuir adsorption analyses were unsatisfactory, even after correction for finite volume fraction of solids. We develop a Langmuir site adsorption model to describe the competitive adsorption of ISS and SDBS in the presence of anchored sites and fit the surface tension data from 84 data points to obtain three parameters: the adsorption equilibrium constants for SDBS ( K s = 760 ± 30 liter mol -1 ) and ISS ( K 1 = 50 ± 12 liter mol -1 ) and the limiting area per molecule (APM = 0.37 ± 0.01 nm 2 ). These parameters are quite consistent with literature values determined in noncompetitive situations. The anchored groups were most significant on smaller particles, and the mobile ISS adsorption was most significant on smaller particles at high volume fraction. The relatively weak attraction of the ISS to the particle surface is due to its low molecular weight (∼400 Da per sulfate group) and results in desorption upon latex dilution.

This chapter focuses on suspension polymerizations in the light of polymer reaction engineering. Engineering of polymerization reactions requires a detailed knowledge of the phenomena that take place in the polymer reactor. This entails a model of the polymerization kinetics and the heat and mass transfer features of the particular polymerization and process. Polymerization reactions are usually complex and a certain degree of mathematical sophistication is required for effective modeling. There are some fundamental differences in the engineering of step-growth and chain-growth polymerizations because of basic distinctions in the mechanisms of these reactions. A propagation reaction in a kinetic chain sequence must be fast or the series of monomer additions will not be long enough to produce high-molecular-weight polymers before the intervention of termination or transfer reactions. This is not generally true for step-growth polymerizations where only an addition reaction is involved and the growth of macromolecules can occur in a series of starts and stops. The major commercial examples of chain-growth polymerizations involve reactions across C=C double bonds to produce polymers with all-carbon backbones. Chain-growth polymerizations that are carried out in the absence of diluents are characterized by high viscosity and poor heat transfer. High-molecular-weight polymer coexists with low-molecular-weight species and monomer, and molecular weight does not increase with conversion. Polymerization reactions can be further classified into batch and continuous processes.

This chapter is devoted to the basic principles of chain polymerizations in which the active centers are free radicals. Free-radical polymerization is the most widely used process for polymer synthesis. It is much less sensitive to the effects of adventitious impurities than ionic chain-growth reactions. Free-radical polymerizations are usually much faster than those in step-growth syntheses, which use different monomers in any case. A monomer must have functionality greater than or equal to 2 in order for polymers to be produced from its reactions. This functionality can be derived from opening of a double bond, opening of a ring, or co-reactive functional groups. The most important functional groups that participate in chain-growth polymerizations are the carbon-carbon double bond in alkenes and the carbon-oxygen double bond in aldehydes and ketones. Chain-growth polymerizations are so called because their mechanisms comprise chains of kinetic events. For successful polymerization, the sequence of reactions must first be initiated by some agent, and monomers must be added consecutively to a growing macromolecule. This chain of events may then be terminated by a reaction that is inherent in the system or by the action of impurities. When a free-radical polymerization is first started, the number of radicals in the system will increase from zero as the initiator begins to decompose according to reaction. The frequency of termination reactions will also increase from zero in the early stages of the polymerization because the rates of these reactions, and are proportional to the square of the total concentration of radicals in the system.

This chapter considers copolymerizations. There are two basic types of heterogeneous polymerizations: The initial reaction medium comprises several phases and polymerization occurs in a heterogeneous system, as in emulsion and suspension reactions, and the reaction medium is initially homogeneous and the polymer forms a separate phase as the polymerization proceeds. Examples are dispersion reactions, described below, and polymerization of acrylonitrile. The chapter focuses on dispersion and emulsion polymerizations. In dispersion polymerization process, the monomer and initiator are soluble in the continuous phase and the polymer particles, which precipitate as they are produced, are stabilized against coagulation by dispersants that comprise different segments that are respectively soluble and insoluble in the continuous phase. Dispersion polymerizations have been used successfully as an alternative to solution polymerization of vinyl polymers for application as surface coatings. An emulsion consists of a discontinuous liquid phase dispersed throughout a different, continuous liquid phase. Milk and the sap of the rubber tree are examples of naturally occurring emulsions. The term latex is used also to denote aqueous dispersions of polymers. Most emulsion polymerizations are free-radical reactions. The main difference from alternative free-radical polymerizations, such as those in bulk, solution, and suspension systems, is that the propagating macroradicals in emulsion reactions are isolated from each other. Encounters between macroradicals are hindered as a consequence, and termination reactions are less frequent than in comparable systems in which the reaction mixture is not subdivided. Emulsion polymerizations thus often yield high-molecular-weight products at fast rates when suspension or bulk reactions of the same monomers are inefficient.

Diffusion of small molecules in polymers is an old topic of significance to a wide variety of industrial processes. In this chapter, various diffusion mechanisms of such a process and the corresponding diffusion coefficients are reviewed. In particular, the Rouse and Zimm models, which are developed based on the concepts of Brownian motion and Langevin dynamics for unentangled polymers, are discussed. The free volume theory is also considered.

This chapter reviews the essentials of practical techniques for measuring average molecular weights and characterizing molecular weight distributions. Most of the procedures for measuring molecular weight rely on colligative solution properties. These properties include osmotic pressure, boiling point elevation, freezing point depression, and vapor pressure lowering. At given solute concentration, these relations show that the effect of the dissolved species on the chemical potential of the solvent decreases with increasing solute molecular weight. Membrane osmometry is the most sensitive and accurate colligative property technique. Two other techniques that are also used to measure molecular weight are not colligative properties in the strict sense. These are based on end-group analysis and on vapor phase osmometry. Both methods, which are limited to lower molecular weight polymers, are described in this chapter. The practical range of molecular weights that can be measured with membrane osmometry is approximately 30,000 to one million. Osmometers consist basically of a solvent compartment separated from a solution compartment by a semipermeable membrane and a method for measuring the equilibrium hydrostatic pressures on the two compartments. Osmotic equilibrium is not reached quickly after the solvent and solution first contact the membrane. Other colligative methods must be employed in range below about 30,000, and vapor pressure lowering can be considered in this connection.

This chapter discusses various polyolefin processes and examples. The active site in chain-growth polymerizations can be an ion instead of a free radical. Ionic reactions are much more sensitive than free-radical processes to the effects of solvent, temperature, and adventitious impurities. Successful ionic polymerizations must be carried out much more carefully than normal free-radical syntheses. Consequently, a given polymeric structure will ordinarily not be produced by ionic initiation if a satisfactory product can be made by less expensive free-radical processes. Styrene polymerization can be initiated with free radicals or appropriate anions or cations. Commercial atactic styrene polymers are, however, all almost free-radical products. Particular anionic processes are used to make research-grade polystyrenes with exceptionally narrow molecular weight distributions and the syndiotactic polymer is produced by metallocene catalysis. Cationic polymerization of styrene is not a commercial process. It is well known in organic chemistry that the reactivities of these various forms of organic ions and counterions may differ significantly, and it is thus not surprising that the same influences are detectable in ionic polymerizations. In general, propagation rates are higher the more the inacroion and its counterion are separated. Steric control of the polymer microstructure is greater, however, when the macroion and the counterion are associated. Rates of ionic polymerizations are by and large much faster than in free-radical processes. This is mainly because termination by mutual destruction of active centers occurs only in free-radical systems.

Polymers are in general use because they provide good mechanical properties at reasonable cost. The mechanical properties of polymers are not single-valued functions of the chemical nature of the macromolecules. They will vary also with molecular weight, branching, cross-linking, crystallinity, plasticizers, fillers and other additives, orientation, and other consequences of processing history and sometimes with the thermal history of the particular sample. When all these variables are fixed for a particular specimen, it will still be observed that the properties of the material will depend strongly on the temperature and time of testing compared to metals. Although the mechanical response of macromolecular solids is complex, it is possible to gain an understanding of the broad principles that govern this behavior. Sections of polymer chains must be capable of packing together in ordered periodic arrays for crystallization to occur. This requires that the macromolecules should be fairly regular in structure. Random copolymerization will prevent crystallization. A melting temperature range is observed in all semi-crystalline polymers, because of variations in the sizes and perfection of crystallites. The crystal melting point is the highest melting temperature observed in an experiment like differential scanning calorimetry. High-molecular-weight flexible macromolecules do not crystallize completely. When the polymer melt is cooled, the crystallites will be nucleated and start to grow independently throughout the volume of the specimen.

Both the mechanical properties of solid thermoplastics and their processing behavior at elevated temperatures depend critically on the average size and the distribution of sizes of macromolecules in the sample. This is one reason why the plastics market contains different grades of each polymer. All varieties are often chemically identical, but some of their molecular-weight-dependent properties may differ enough that the polymers cannot be interchanged economically. If the molecular weight of the polymer is too high, the material will not flow out enough to weld well under normal sealing conditions. On the other hand, if the molecular weight is too low, , the plastic may suffer excessive thinning, resulting in a weak weld area or show-through of fabric backing. Molecular weights are not often measured directly for control of production of polymers because other product properties are more convenient experimentally or are thought to be more directly related to various end uses. This chapter reviews the fundamentals of small particle statistics as these apply to synthetic polymers. This is mainly concerned with the use of statistical moments to characterize molecular weight distributions.

This chapter reviews the original classification of polymers and shows why it is no longer generally applicable. It summarizes the current accepted meaning of the terms addition and condensation polymers, and then turns to a useful, alternative classification which focuses on polymerization processes rather than the products of such processes. There are many possible ways to classify polymers. Each may be useful depending on the interests of the classifier. Examples include typing according to the source of the product (naturally occurring polymers, entirely synthetic macromolecules, or those derived by chemical modification of naturally occurring polymers), chemical structure (polyolefin, polyamide, et cetera.), polymer texture during use (rubbery, glassy, partially crystalline), area of application (adhesive, fiber, etc.), and so on. An important classification divides macromolecules into addition and condensation polymers. This distinction was made by W.H. Carothers, who invented nylon-6, 6 and made many fundamental contributions to our knowledge and control of polymerizations. A condensation polymer is one in which the repeating unit lacks certain atoms which were present in the monomer(s) from which the polymer was formed or to which it can be degraded by chemical means. In addition polymers, by contrast, the recurring units have the same structures as the monomer(s) from which the polymer was formed. Examples are polystyrene (1-1), polyethylene (1-3), styrene-maleic anhydride copolymers (1-26), and so on.

This chapter reviews some basic concepts and definitions of terms used in the polymer literature. Polymer technology is quite old compared to polymer science. For example, natural rubber was first masticated to render it suitable for dissolution or spreading on cloth in 1820, and the first patents on vulcanization appeared some twenty years later. Polymer means “many parts” and designates a large molecule made up of smaller repeating units. They generally have molecular weights greater than about 5000 but no firm lower limit need be defined since the meaning of the word is nearly always clear from its use. The word macromolecule is a synonym for polymer. The repeating unit of a linear polymer is a portion of the macromolecule such that the complete polymer might be produced by linking a sufficiently large number of these units through bonds between specified atoms. Polymers can be usefully classified in many ways, such as by source of raw materials, method of synthesis, end use, and fabrication processes. The distinction between elastomers, fibers, and plastics is most easily made in terms of the characteristics of tensile stress-strain curves of representative samples. The parameters of such curves are nominal stress (force on the specimen divided by the original cross-sectional area), the corresponding nominal strain (increase in length divided by original length), and the modulus (slope of the stress-strain curve).

There has been heightened interest in using biopolymers in recent years due to the increasing concern with the sustainability of using petroleum-based polymers. This chapter reviews the current effort of chemically modifying various natural polymers obtained from renewable sources into useful products and of synthesizing biopolymers through polymerization of bio-monomers. Biopolymer blends, biocomposites and the future of bioplastic products are briefly discussed.

Commercial ethylene–octene linear low-density polyethylenes (LLDPEs) were reactively extruded with low levels of a peroxide [2,5-dimethyl-2,5-di(t-butylperoxy)hexane] to modify polymer molecular structure and processing properties. Peroxide levels were kept low to avoid crosslinking. This article reports the effects of this reactive extrusion on viscoelastic properties. Rheological properties are more sensitive than are molecular structure characteristics to the changes produced by reactions of very low peroxide concentrations. Complex viscosity increases are seen, especially at low frequencies. Shear-thining behavior is also accentuated. The crossover between G′ and G″ moves to lower frequencies. A modified Cole–Cole presentation of these data shows that the elastic component is more predominant for extrusion-reacted materials. Peroxide-modified materials all have higher Bagley end correction values than those of barefoot resins. The former exhibit lower power indices (more shear thinning). All these properties indicate more long-chain branching and higher melt elasticity. However, die swell decreased as a function of peroxide concentration. Peroxide treatment results in an enhancement of elongational viscosity, both under isothermal and cooling conditions, along with a decrease in drawdown ability. The rheological changes parallel those reported earlier in molecular characteristics but are more sensitive and suitable to evaluate the effects of reactive processing. The effectiveness of the reactive extrusion process for improving processability of LLDPEs depends critically on the extrusion conditions. © 1995 John Wiley & Sons, Inc.

The emulsion polymerization of large monodisperse MMA/BA copolymer particles by surfactant-free means at high solids content is described. A series of sequential seeded growth reactions were used to obtain monodisperse particles with diameters of at least 3 microns. A combination of an oil-soluble initiator, AIBN, and a water-soluble initiator, ammonium persulfate (AP), was used. The AIBN initiated the bulk of the polymerization and the AP provided the stabilization for the polymer particles through its charged endgroups. The amount of AP initiator to be used in each growth step was calculated empirically using a criterion of a constant ratio of moles of sulfate groups to the polymer particle surface area. © 1995 John Wiley & Sons, Inc.

Mechanical and rheological properties of blends of polypropylene (PP) and linear polyethylene (PE) were studied, with particular attention to the effects of aging of such mixtures. These two olefin polymers are basically incompatible. In PP-rich blends, addition of highdensity PE (HDPE) causes only a slight decrease in tensile properties and impact resistance of injection-molded specimens. In all cases, annealed specimens have higher moduli and lower impact strength than as-molded products. While none of these changes are very drastic, the addition of small amounts of HDPE was observed to result in a serious decrease of gate-region impact resistance of thin-walled moldings. Blends with 10–20% HDPE exhibited an unexpected interaction in tensile, thermal, and melt-flow properties as well as in crystallization behavior. © 1995 John Wiley & Sons, Inc.

In this article, we report the results of molecular dynamics simulations for models of poly(ethylene), isotactic poly(propylene), and mixtures of the two to understand the possible origin of melt state incompatibility. At temperatures well above the crystal melting points of the homopolymers, isolated poly(ethylene) and poly(propylene) chains formed high-density collapsed states. The poly(ethylene) and poly(propylene) chains were found to segregate into distinct domains even when the initial state was highly interpenetrating. The poly(ethylene) phase shows a significant amount of local order at the simulation temperature. In the case of poly(propylene), some helical segments were found in the collapsed structure. We speculate that the immiscibility of these two nonpolar polymers, in the melt state, may be due to the differences in local morphology that are present in the melt state.

A “surfactant-free” emulsion polymerization of methyl methacrylate was conducted at 80°C with ammonium persulfate initiator. The water-soluble MMA-based oligomers re-maining were isolated and analyzed both as-produced and after hydrolysis. NMR analysis revealed that the mole ratio of MMA repeat units to sulfate end groups in the former is 6.5. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and gel permeation chromatography both showed that the mean number of MMA repeat units comprising the extracted material is 8–9. The hydrolysis procedure rids the oligomers of their terminal groups and converts some of the methyl ester groups to acid, greatly sim-plifying the mass spectrum. This appears to be the first direct measurement of “surfactants” produced in situ from monomer and persulfate initiator. © 1995 John Wiley & Sons, Inc.

The emulsion polymerization of large MMA/BA copolymer particles with narrow particle size distributions and core-shell structure is described. A series of sequential seeded growth emulsion polymerizations were used to obtain monodisperse particles with diameters of at least 3 μm, at 30% solids contents. Because the core and shell polymers used here were chemically similar, core-shell structures could not be verified by differential staining tech-niques. Core-shell structure was demonstrated by minimum film-forming temperature studies and by scanning electron microscopy in conjunction with energy dispersive x-ray analysis, using chlorine-labeled core polymers. © 1995 John Wiley & Sons, Inc.

It is well known that the amount of surfactant must be carefully controlled during starve-fed emulsion polymerization processes. Too little surfactant leads to emulsion instability and coagulation, while too much surfactant leads to secondary particle formation. Although these relationships are qualitatively understood in the art, there is little quantitative basis to guide the synthetic chemist, especially in multistep starve-fed emulsion polymerization processes to make larger supermicron particles. We have developed a method, which will be described in a companion article, to control the surfactant level by monitoring the surface tension during polymerization. In order to quantitatively predict how much surfactant to add at any given time, one needs to know in advance the adsorption characteristics of the soap. Further complicating the matter is the formation of “in situ” or oligomeric surfactant during polymerization with aqueous initiators such as ammonium persulfate. This work demonstrates how to prepare surface-active oligomers and how to make latex particles using them as surfactant. First, we established the mass balance for the initiator-derived sulfate groups in seed latexes by conductometric, potentiometric, and iodometric titrations. Based on the characterization of seed latexes, a method for determining the effective sulfate concentration has been developed. When surface-active oligomers were used as the only surfactant, we obtained a series of monodisperse, supermicron copolymer latex particles with diameters up to 3.22 μm. This is a similar result to that obtained with a commercially made anionic surfactant. © 1995 John Wiley & Sons, Inc.

In a multidetector SEC system it is necessary to match the outputs of the detectors that sense eluant concentration with those that are molecular weight dependent. There are several methods to accomplish this. The most common is to determine the interdetector volume or time difference between detectors. The difficulty with this technique is that with detector systems where the column eluent is split, the interdetector time difference varies with solution viscosity. Two new techniques were implemented on a high temperature SEC/DRI/LALLS/VISC system which take into account changes in solution viscosity due to high molecular weight polymers. Both techniques use calibration curves of hydrodynamic volume versus elution volume for the different detectors to determine the instantaneous time difference between the detectors as it varies with solution viscosity. Either method provides the correct interdetector time lag information. We note also that in multidetector systems the configuration of the detectors should be such as to maximize solvent flow rates through each detector and hence to minimize band broadening effects. © 1995 John Wiley & Sons, Inc.

A commercial linear low density polyethylene (1-hexene comonomer) was reacted with dicumyl peroxide at appropriate levels to initiate chain extension without crosslinking. The peroxide modification was carried out both in the presence and absence of a common hindered phenolic antioxidant. Thermal treatment of the resin was also completed with and without antioxidant present to evaluate the contribution of classical thermooxidative degradation to the peroxide modification process. In terms of pure thermal degradation, the phenolic antioxidant was found to be effective in prevention of reduction of molecular weights at the high end of the molecular weight distribution, as reflected in changes in M z . At the low end of the distribution, the opposite effect is seen, where the removal of the antioxidant actually allows an increase in the value of M n , through chain coupling reactions. Processing of the resin with addition of the organic peroxide significantly increases the molecular weights of both antioxidant containing and antioxidant free resins. Increasing the reaction temperature increases the rate of degradative chain scission and nonenlarging disproportionation reactions. At low temperatures, the presence of the antioxidant acts negatively with respect to chain enlargement reactions, and an overall reduction is seen in the efficiency of the peroxide in its role of increasing molecular weights. As the reaction temperature is increased, chain scission reactions become more predominant, and the antioxidant free resin shows very substantial decreases in molecular weight. Retention of the antioxidant at higher reaction temperatures is beneficial to maintaining high molecular weights, both with and without peroxide present. As in the peroxide free case, branching is essentially unchanged by the absence of antioxidant in peroxide modification. © 1995 John Wiley & Sons, Inc.

The dominant species in the early stages of an emulsifier-free emulsion polymerization of styrene has been found to be an oligomer of two to three monomer units using a novel trapping technique. This involved the intercalation of charged primary oligomers between the layers of a hydrotalcite, [Mg4Al2(OH)12]2+[A]2- (where A = dianion). Hydrotalcites are an important class of lamellar, inorganic compounds whose interlayer spacing can be mod-ified by anion exchange. Our approach first involved preparing a hydrotalcite precursor in which the layers were propped apart by an organic dianion (terephthalate = TA). This material was then used to capture the negatively charged polystyrene oligomers from the emulsion polymerization reaction mixture. We found that TA was rapidly ion-exchanged for the charged oligomers. The resulting pillared hydrotalcite material was characterized using XRD and SEC. We found that the interlayer spacing between the hydroxide layers increased to 23.2 A on exposure to the emulsion reaction mixture. This represents an interlayer expansion of 18.3 A (after subtraction of the hydroxide layer contribution), which is cnsistent with intercalation of oligomers with two to three monomer units arranged in a bilayer. This size estimate was confirmed by the results of size exclusion chromatography. © 1995 John Wiley & Sons, Inc.

Inverse gas chromatography of alkanes has been used to assess the degree of fusion of rigid polyvinyl chloride (PVC) extrudates. The relative degree of fusion is provided by ratios of the retention volumes of the alkane probe molecules in the bulk of the PVC. This method is nonintrusive and can be used to measure degree of fusion quantitatively if a completely fused reference material can be provided. The present analysis involves evaluations of the contributions to the overall retention volume of the alkane probes, resulting from adsorption and from bulk diffusion. The diffusivity of the alkanes was clearly dependent on the processing history of the PVC extrudate. A direct variation was observed between the size of the diffusing molecules and their residence time as adsorbates on the polymer surface. © 1995 John Wiley & Sons, Inc.

This review focusses on processes in which emulsion polymerizations are carried out in stages so that previously formed particles are either overcoated in subsequent polymerization stages or engulf the second and later stage polymers. These products are often called “core-shell” particles. Basically, the most stable state of the final system is the one with the lowest net interfacial energy. In the case of a two-stage emulsion polymerization there can be three interfacial tensions to consider. Several mutually consistent, effective thermodynamic treatments have been published. At present, they serve primarily to predict when the morphology of multi-stage polymerization products may not be a simple reflection of the synthesis sequences. It is possible, and frequently desirable, however to produce particle structures that appear at first glance to be thermodynamically forbidden. This is achieved either by changing the surface characteristics of a polymer from those of the bulk material or by employing kinetic factors to anchor energetically unprofitable morphologies. This paper summarizes methods of both types that have been reported to control the texture of structured latex particles in order to produce designed morphologies.

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry was used to characterize nonionic, anionic and cationic surfactants. Rapid mass analyses can be achieved with proper choice of matrix material. The technique is suitable for development as a routine analytical tool.

The solubility properties of (hydroxyethyl)- and (hydroxypropyl)cellulose are examined using molecular models and molecular dynamics simulations. The calculations were performed by building models of both the amorphous condensed state and the vacuum state. The condensed state models were constructed with single chain conformations in periodic cells with backbone torsion angles populated according to RIS determined for the single chains. For HPC-253, a (hydroxypropyl)cellulose for which experimental results have been published recently, we found good agreement between the simulations and experimental results. The simulations predict that the three-dimensional solubility parameters for this class of surfactants are dominated by the contributions from polar and hydrogen bonding interactions

Low-density polyethylene (LDPE) and also linear low-density polyethylene (LLDPE) resins can be characterized by the degree of strain hardening and down-gaging during elongation. A new method for the determination of the apparent elongational flow characteristics is presented. In a small scale apparatus, a molten monofilament is stretched under nonisothermal conditions similar to those found in tubular film extrusion. Measurement of resistance to elongational flow and apparent elongational strain rates permit the comparison of the process-ability of different resins under specified conditions. The effect of melt temperature and extension ratio are examined. The importance of the molecular structure of both LDPE and LLDPE resins on these properties is also outlined.

Mixtures of film grade LLDPE, LDPE, and PP may occur in recy cle streams. Household plastic films collected through a recycling program were found to contain less than 2 per cent PR The present study concerns the effects of minor amounts of polypropylene on the film properties of such mix tures. Polypropylene enhances film stiffness slightly and has no deleterious ef fect on other PE film properties at levels up to 5 per cent PP However, consider able degradation of tear and impact strength occurs with higher polypropylene concentrations. These problems are alleviated by addition of a vinyl acetate EVA film grade polymer of 28% VA content, which will permit PP levels ap proaching 20 per cent without negative impact on film properties. Good proper ties were observed in 85/15 LLDPE/PP mixtures by addition of 5 per cent of this EVA, based on the total polyolefin content. This report gives a detailed descrip tion of the mechanical properties of the extruded films.

A comprehensive model of film formation that was proposed in an earlier article by the present authors was evaluated for latexes having a wide range of physical properties. In this model, capillary force and interfacial forces act in tandem to promote film coalescence. The emulsion polymers studied were all based on poly(methyl methacrylate-co-butyl acrylate). The viscoelastic nature of the copolymer was varied by the addition of molecular weight modifiers (CBr4 chain-transfer agent and ethylene glycol dimethacrylate cross-linking monomer) during synthesis. In contrast to the simplifications of the previous analysis, the experimental system was well characterized. The effect of water plasticization was investigated. The surface energetics of the system were studied. Also, drying kinetics were considered. Results showed that the degree of coalescence ranged from complete fusion to superficial fusion. The minimum film temperature was found to increase with increasing polymer rigidity, as is expected. Comparison of the model predictions and experimental observations validated the model. © 1994 John Wiley & Sons, Inc.