Mimi Y. Keating

Thermal fractionation of ethylene polymers in packaging applications

Abstract In the past 5 years, new ethylene copolymers have found their way into a wide variety of applications. The new polymers have a unique combination of properties not typically found in conventional elastomeric or thermoplastic ethylene polymers. The characterization of these polymers by new and improved analytical techniques is of great interest. We describe a thermal fractionation method which sorts the crystalline ethylene sequence lengths of the polymer into groups. The ethylene lengths are estimated using melting points of known hydrocarbons. Three statistical terms are introduced to describe ethylene sequence length distribution: the arithmetic mean L n , the weighted mean L w and the broadness index L w L n . We demonstrate the technique by comparing ethylene polymers produced with different catalyst systems, decipher ethylene components in unidentified ethylene blends and track changes in crystalline fractions with grafting.

Evaluation of the comonomer distribution in ethylene copolymers using DSC fractionation

Abstract The microstructures of ethylene random copolymers are evaluated using DSC fractionation by the degree of crystallinity. This method is based on the same principle of separation as temperature-rising elution fractionation (TREF) which uses a crystallization/dissolution procedure; however it does not physically separate the fractions. The resulting copolymers are sorted by crystallite sizes; the crystallite size and lamellae thickness are directly related to the ethylene block length. Copolymers including E/VA, E/nBA, E/nBA/CO, E/nBA/VA, E/MAA, E/AA and LLDPE have been analyzed by DSC fractionation to elucidate structural differences. We see that (a) specimens with high comonomer content are less crystalline; the ethylene segments are shorter, (b) the comonomer type makes a difference in fractionation if H-bonding is involved, (c) the narrow comonomer distribution, i.e. narrow ethylene-segment-length distribution, has fewer DSC fractions; the broad distribution has more fractions, (d) fractionation by crystallinity is affected if the molecular weight/viscosity is very high, (e) branch content reduces the crystallinity and shortens the ethylene blocks, and (f) there is no direct relation to molecular weight distribution or comonomer content.

Effects of the phase-separated melt on crystallization behavior and morphology in short chain branched metallocene polyethylenes

Abstract Some metallocene catalyst synthesized short chain branched polyethylene (SCBPE) samples have been found to possess at least intermolecular heterogeneity in the SCB. As-received SCBPE samples are molecularly homogeneous in the isotropic melt. However, phase separation due to the intermolecular heterogeneity can be found via molecular segregation processes induced by multiple-step isothermal crystallization experiments. When the phase-separated SCBPE samples are reheated above their melting temperatures, the phase-separated (heterogeneous) melt is maintained for an extended period of time (at least 20 h at 150°C). Neither phase mixing in the melt nor significant changes in molecular weight, molecular weight distribution, comonomer content, or sequence have been found during the high-temperature treatment. Comparisons of overall crystallization kinetics and morphology of the SCBPE samples obtained from the homogeneous and heterogeneous melts exhibit substantially different behavior which indicates t...

A.C. dielectric and TSC studies of constrained amorphous motions in flexible polymers including poly(oxymethylene) and miscible blends

Poly(oxymethylene) ( POM ) and its miscible blends were studied by multifrequency A.C. dielectric and thermally stimulated currents (TSC). The blends contained small amounts of either poly(vinyl phenol), which is a high glass transition (T g diluent, or a styrene-co-hydroxy styrene oligomeric low T g diluent. The variation of the 10°C β transition with blend composition proves that it is the glass transition, and that the -70°C y transition is a local motion. Dielectrically the P transition is very weak in pure POM even in fast-quenched samples. The TSC thermal sampling method also detected two cooperative transitions, and β, in POM and its blends, and was used to directly resolve the transition into low and high activation energy components. If one considers the contribution of exclusion of the diluents from the crystal lamellae, it is shown that the blends behave like typical amorphous blends as a function of concentration. The effect of crystals on amorphous motions is examined in light of comparison with van Krevelens 37 predictions of an amorphous T g , and the transitions in POM are contrasted with those for other semicrystalline polymers.

Glass transition, crystallinity, resin stiffness, and branch distribution in metallocene and Ziegler-Natta ethylene 1-olefins

The physical properties of commercially obtained metallocene and Ziegler-Natta (ZN) polyethylenes (PEs) were correlated to their backbone structure (i.e., the composition, ethylene sequence distribution, and short-chain branch length) using dynamic mechanical analysis and thermal fractionation. The uniformity or heterogeneity of the 1-olefin branch distribution on the ethylene backbone profoundly controls the mean crystalline ethylene segment lengths, the degree of crystallinity, the beta transition temperature and intensity, the plateau modulus, and the presence or absence of the alpha transition.

Conformation of ethylene/propylene copolymers (random or block) as seen by 13C NMR, IR and thermal methods

Abstract Selected random and block ethylene/propylene (E/P) copolymers have been investigated thoroughly using 13 C NMR, IR, DSC and dynamic mechanical analysis (DMA). The ethylene content in the range 1–6% in random E/P copolymers can be quantified with good agreement by NMR relative intensity of PEP triad, by IR band intensity of the isolated ethylene units, and by DSC melting peak depression. The toughened polypropylenes containing E/P blocks show the expected “run” sequence of five or more methylene groups of intense PEE and EEE triads sequence in NMR and in IR bands. The glassy-rubbery transitions of the two-phase-structure block E/P copolymers are clearly demonstrated by DMA damping signals.

Annealing effect on semi-crystalline materials in creep behavior

Three glass reinforced polymers of Nylon 66, PET and HT-Nylon with glass transitions of 58, 72, and 132°C are selected for checking the necessity of high temperature annealing and degree of its impact on creep strains. The improvement after annealing depends on the polymer type of whether it is fast or slow crystallizing at molding process and whether it has hydrogen bonded sheets. The physical property changes observed of these materials before and after annealing support the explanation of crystal reorganization through crystallization, free volume reduction through densification and crystal perfection through better chain packing. The prediction of long-term creep strains up to 10 years using time-temperature superposition technique has been the practice for last 13 years in our laboratory. The accuracy of the prediction are shown in confidence level of about 90%.

Structural Development During Thermal Fractionation of Polyethylenes

In this study, the stepwise isothermal crystallization or thermal fractionation of Ziegler—Natta and metallocene based polyethylenes (ZN-PE and m-PE) with two kinds of branch lengths (ethyl and hexyl) and branch compositions were studied using simultaneous synchrotron small-angle X-ray scattering (SAXS)/wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC). The crystal long period and the invariant were determined by SAXS, and the variations of crystal unit cell parameters and the degree of crystallinity were determined by WAXD. The arithmetic mean length (Ln), the weightedmean length (Lw) and the broadness index (Lw/Ln) of the studied polyethylenes were previously determined by DSC. Results from these studies were interpreted using the model of branch exclusion, which affects the ability of the chain-reentry into the crystal phase. Multiple SAXS peaks and step-change in crystallinity change (WAXD) were seen during heating, which corresponded well with the crystal thickness distribution induced by stepwise crystallization. The effects of the heterogeneity of the 1-olefin branch length and the distribution on the crystal long period and the invariant as well as the degree of crystallinity were discussed.

Dynamic mechanical characterization of relaxations in poly(oxymethylene), miscible blends, and oriented filaments

Abstract Multifrequency dynamic mechanical analysis (DMA) data were obtained for molded poly(oxymethylene) (POM) and its blends from-150°C to 150°C. Because of the high crystallinity, the assignment of the glass transition in POM has been controversial in the literature. Low and high glass transition temperature (T g) phenolated compounds, including poly(vinyl phenol), were found to be miscible with POM. The shift of the β transition in the POM blends favors an assignment of the β transition detected at −3°C(1 Hz), not the −80°C γ transition, as the T g in semicrystalline POM because the latter is invariant with diluent. The peak at the β transition in pure POM is weak and can only be seen clearly by DMA measurements on samples that have not “aged” at ambient temperature. This is further evidence that the β transition arises from a cooperative glass-transition-like motion. The γ transition is not influenced by aging because it is due to a concerted localized main chain motion. The β transition of an orien...

Thermal conductivity of polymer melts

Abstract The Du Pont 910 DSC cell and base have been modified for the measurement of steady-state heat flux and temperature gradients through molten polymeric specimens. This is a comparative method for thermal conductivity determination. The calculation for polymer melts is based on the measured thermal quantities and a calibration factor obtained from reference materials. The measurements at several steady-state temperatures can be carried out consecutively. Furthermore, the thermal diffusivity k can be obtained from the relationshipκ = λ/ρCp, where λ is the thermal conductivity, ϱ is density, and Cp is specific heat. Construction of sample holders and modifications of the DSC cell and base are described.