Effect of dispersity on the Tll (>Tg) transition in polystyrene

The presence of a high temperature (>T_g) relaxation in amorphous polystyrene has been investigated further. In the previous work,¹ the techniques of differential thermal analysis (DTA) and torsional braid analysis (TBA) were employed to study polystyrene as a function of “monodisperse” molecular weight. The occurrence of the T_ll transition appeared to be associated with the attainment of a critical viscosity level with also corresponded with a free volume level. An entanglement network developed at a critical value of molecular weight, M_c, giving a break in the T_ll-versus-M plots. The present work deals with the influence of dispersity on the T_ll transition, below and above M_c. A series of binary blends of “monodisperse” anionically polymerized polystyrenes with systematic changes in M?_n and heterogeneity index (M?_w/M?_n) was tested by TBA. The results show that when both components have molecular weights below M_c, single and average values of T_g and T_ll are observed which are linearly related to M?_n^?1, as predicted by free volume arguments. Although a single T_g is observed when one component has a molecular weight above and the other has a molecular weight below M_c, the components appear to undergo the T_ll relaxation independently. The results indicate that both the glass transition and the T_ll transition are basically governed by the same type of molecular motion but at different length ranges.     

The Tll (>Tg) transition of atactic polystyrene

Torsional braid analysis (TBA) (～0.3 Hz) and differential thermal analysis (DTA) data are presented for the temperature for the region 0–200°C for two series of atactic polystyrenes with narrow molecular weight distributions: (a) anionic series, M?_n = 600–2×10⁶, M?_w/M?_n ? 1.1; (b) fractionated thermal series, M?_n = 2,000–1.1×10⁵, M?_w/M?_n < 1.25. Preliminary results on bimodal blends are also reported. Heating and cooling cycles were employed with TBA; only the heating mode was used with DTA. In addition to a dynamic mechanical loss peak at T_g, a higher temperature loss peak was also found. Designated the T_ll or liquid–liquid transition (relaxation), its temperature is 1.1 to 1.2 T_g (°K) for polymers with molecular weight below the critical molecular weight (M_c) for chain entanglements. Above M_c ? 35,000, it rises steeply, being ?200°C for M?_n = 110,000. The common dependence of T_g and T_ll on M?_n^?1 below M_c suggests a common molecular origin. The two facts, (a) that T_ll > T_g and (b) that T_ll reflects chain entanglements, further suggest that T_ll involves a longer chain segment length and possibly the entire molecule. Comparison of T_ll versus log M plots with T versus log M isoviscous state plots based on zero-shear melt viscosity data from the literature implies that T_ll measured by the TBA technique corresponds to an isoviscous state of 10⁴–10⁵ poises. The employment of narrow molecular weight polymers is presumably responsible for both the linear variation of the T_ll transition with M?_n^?1 (which suggests a free volume basis for the relaxation) and the form of the variation of the T_ll transition with log M (which suggests an isoviscous basis for the relaxation). The sharpness of the T_ll loss peak by TBA decreases with increasing molecular weight and dispersity. The DTA endothermic event corresponding to T_ll is clearly related to the occurrence of flow since the fused films which result from heating granules to 200°C and cooling to R.T. do not reveal a T_ll on reheating. If a fused film is crushed, a T_ll event is observed on heating. For bimodal blends with M?_n < M_c for both components, the T_ll transition was averaged; with one component less than and one greater than M_c, the T_ll transitions of the components appeared to occur independently at temperatures corresponding to those of the isolated components. In accordance with Ueberreiter and Orthmann, T_g appears to separate a glassy state from a fixed liquid state, whereas T_ll separates the fixed liquid from a true liquid state. Possible molecular interpretations for the T_ll process are discussed. Systematic bodies of data from the literature which indicate the presence of the T_ll process in other polymers are summarized.     

Reinterpretation of the DSC data for atactic polystyrenes by Stadnicki et al.

Reexamination of published DSC traces for first and second heatings of atactic polystyrenes with M?_n from 2050 to 1.99 × 10₆, M?_w/M?_n ～ 1.1, is discussed in terms of some newly enunciated principles of calorimetry for the liquid state of atactic polymers. These traces were originally stated to reveal only an endothermic peak above T_g, identified with T_ll at M?_n < ca. 10⁵. Three types of behavior are now distinguished: (1) First heating, M?_n < M_c (the entanglement molecular weight), an endothermic slope change attributed to T_u, followed by an exotherm at T_exo ascribed to melt flow and/or wetting of the DSC pan; (2) first heating for M?_n above M_c, only an endothermic slope change; (3) second heating, all molecular weights, only an endothermic slope change for T_ll and, in some cases, T_lp. T_ll thus defined reaches an asymptotic limit when plotted against log M?_n, just as T_g does. Texo begins to level off, and then, starting at M_c, increases without limit. T_ll is an isofree volume state as is T_g; T_exo is an isoviscous state and hence a pure relaxation process. DSC traces on first heating at M?_n < 10⁵ provide the first known experimental evidence for both the transitional and the relaxational aspects of T_ll in the same experiment. The role of zero shear melt viscosity, η₀, in DSC is noted: T_ll depends only on M?_n; T_exo on η₀ and hence on M?_w. The T_ll temperature is independent of physical form of the specimen: powder, pellet, film. DSC studies on bimodal blends of PS show T_ll uniquely dependent on M?_n, while the T_f – (T_exo) exotherm depends on M?_w. Such blends can eliminate interference between the T_ll endotherm and the T_f exotherm. Exotherms using weighted pan lids and exotherms from sintering are also presented. DSC evidence for T_lp > T_ll is indicated.     

Synthesis,characterization, and properties of novel polyisobutylene-based urethane model networks

This paper concerns the synthesis, characterization, and physical properties of novel polyisobutylene (PIB)-based urethane model networks prepared from diphenylmethane diisocyanate (MDI) and three-arm star PIBs capped with ? CH₂OH end groups (PIB(CH₂OH)₃). The PIB(CH₂OH)₃ starting materials were produced by the inifer method in the M?_n = 13,550–27,000 range. The best networks were obtained with NCO : OH = 1. Solvent extractions showed uncatalyzed network formation to be essentially complete and swelling studies indicated the expected architecture, i.e., M?_c = 2M?_n/3 and virtual absence of dangling chains. Stannous octoate was found to increase the rate of network formation in the absence of side reactions (i.e., allophanate formation), while other catalysts also accelerated undesirable reactions. The effect of molecular weight between crosslinks (M?_c) on network physical properties has been studied in the M?_c = 900–18,500 range. The tensile strength increases with decreasing M?_c up to a limiting value after which it sharply declines. Elongations at break decrease monotonously with decreasing M?_c. Low temperature tensile studies show higher tensile data at ?20°C, and, surprisingly, a retention of elongations at break. The T_g's decrease with increasing M?_c's until a plateau is reached at ?73°C. Hysteresis is fairly constant and permanent set is constant and low. The PIB-based urethane networks exhibit excellent hydrolytic stability, negligible moisture absorption, and outstanding heat-aging stability, far beyond what is expected for a polyurethane. Conceivably the thermal deblocking of the urethane group may be reversible (? NH? COO? CH₂? ? ? NCO + HOCH₂? ) because the isocyanate that arises in the highly hydrophobic PIB matrix recombines with the alcohol, and cannot react with moisture as in conventional urethane networks.     

The role of network architecture on the glass transition temperature of epoxy resins

A series of epoxy networks were synthesized in which the molecular weight between crosslinks (M_c) and crosslink functionality were controlled independent of the network chain backbone composition. The glass transition temperature (T_g) of these networks was found to increase as M_c decreased. However, the rate at which T_g increased depended on crosslink functionality. The dependency of M_c on T_g is well described by two models, one based on the concept of network free volume while the other model is based on the principle of corresponding states. Initially, neither model could quantitatively predict the effect of crosslink functionality in our networks. However, our tests indicated that both the glass transition and the rubbery moduli of our networks were dependent on M_c and crosslink functionality, while the glassy state moduli were independent of these structural variables. The effect of crosslink functionality on the rubbery modulus of a network has been addressed by the front factor in rubber elasticity theory. Incorporation of this factor into the glass transition temperature models allowed for a quantitative prediction of T_g as a function of M_c and crosslink functionality. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 387–395, 1997     

Structure and viscoelastic properties of epoxy resins prepared from four-nuclei novolacs

The relation between the structure and the viscoelastic properties of seven kinds of epoxy resins was studied. Seven tetraglycidylethers were synthesized from four-nuclei novolacs in which the positions of methylene linkage or number of kind of substituents were different. These epoxy compounds were cured with diaminodiphenylmethane as a hardener. From the viscoelastic properties of the fully cured resins with the hardener, characteristic properties such as glass transition temperature (T_g), average molecular weight between crosslinking points (M̄_c), and front factor (ϕ) were obtained. It was concluded that higher linearity in the main chain of epoxy resins gave a cured resin with a higher T_g, a smaller M̄_c, and a larger ϕ.     

Effects of polycaprolactone molecular weights on thermal and mechanical properties of polybenzoxazine

Polymer blends of polybenzoxazine (PBA‐a) and polycaprolactone (PCL) of different molecular weights (M_n = 10,000, 45,000, and 80,000 Da) were prepared at various PBA‐a/PCL mass ratios and their properties were characterized. The results from dynamic mechanical analyzer (DMA) revealed two glass transition temperatures implying phase separation of the two polymers in the studied range of the PCL contents. Moreover, a synergistic behavior in glass transition temperature (T_g) was evidently observed in these blends with a maximum T_g value of 281°C compared with the T_g value of 169°C of the PBA‐a and about ?50°C of the PCL used. The blends with higher M_n of PCL tended to provide greater T_g value than those with lower M_n of PCL. The modulus and hardness values of PBA‐a were decreased while the elongation at break and area under the stress?strain curve were increased with an increase of the content and M_n of PCL, suggesting an enhancement of toughness of the PBA‐a. Scanning electron micrographs (SEM) of the sample fracture surface are also used to confirm the improvement in toughness of the blends. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41915.     

In-chain functionalization by alternating anionic copolymerization of styrene and 1-(4-dimethylaminophenyl)-1-phenylethylene

Anionic copolymerizations of styrene (M₁) with excess 1-(4-dimethyl-aminophenyl)-1-phenylethylene (M₂) were conducted in benzene at 25°C for 24h, using sec-butyllithium as initiator. Narrow molecular weight distribution copolymers with M?;_n = 16.1 × 10³ g/mol (M?_w/M?_n = 1.04) and 38.2 × 10³g/mol (M?_w/M?_n = 1.05), and 24 and 38 moles of M₂ per macromolecule, respectively, were characterized by size exclusion chromatography, ¹H NMR spectroscopy and DSC. The monomer reactivity ratio, r₁ = 5.6, was obtained from the copolymer composition at complete consumption of M₁, assuming that the rate constant k₂₂ =0,i.e. r₂ =0. The polymers exhibited T_g values of 128 and 119°C, respectively, which correspond to an estimated T_g = 217°C for the hypothetical homopolymer of M₂.     

Physical properties of perfluoropolyethers: Dependence on composition and molecular weight

Three copolymeric perfluoroethers with the structure CF₃[(OCF₂CF₂)_p(OCF₂)_q] OCF₃, having different p/q ratios, have been fractionated. The fractions obtained have been characterized by Gel Permeation Chromatography and ¹⁹F-NMR. The viscosity η the specific volume v and the glass transition temperature, T_g have been measured by standard techniques for all the above samples as well as for some other perfluorinated polyethers. The temperature dependence of viscosity of the unfractionated samples is described by the W.L.F. equation. The values of f_g (fractional free-volume at T_g) and of a_f (free-volume expansion coefficient) are independent of composition, for p/q ratios from 0.53 to 1.15. The critical molecular weight, M_c, is of the order of 8–9,000. From the molecular weight dependence of specific volume, the contribution to the molar volume of the in-chain CF₂ group and the excess molar free volume of the chain ends have been determined. The limiting value of T_g for an infinite molecular weight polymer was found to depend linearly on the compositional ratio O/C and the extrapolated values for polytetrafluoroethylene and for the homopolymer (CF₂O)_n were found to be respectively 200 K and 120 K.     

Investigation of the Tu(>Tg) relaxation in homopolymers and block copolymers of styrene by torsional braid analysis

Investigation of the T_u (>T_g) relaxation in amorphous polymers of styrene by the technique of torsional braid analysis is reviewed. For the most part the relaxation behaves like the glass transition (T_g) in its dependence on molecular weight, on average molecular weight in binary polystyrene blends, and on composition in a polystyrene homogeneously plasticized throughout the range of composition. Diblock and triblock copolymers also display a T > T_g relaxation above the T_g, of the polystyrene phase. Two results in particular suggest that the T_u relaxation is molecularly based. (1) The T_u temperature is determined by the number average molecular weight for binary blends of polystyrene when both components have molecular weights below M_c. (the critical molecular weight for chain entanglements). (2) Homopolymers, and diblock and triblock copolymers of styrene, have a T > T_g relaxation at approximately the same temperature when the molecular weight of the styrene block is equal to that of the homopolymer.     

Determination of glass transition temperature from dielectric analysis for a series of epoxide oligomers

Dielectric properties have been investigated for a bisphenol-A type epoxide oligomer, whose weight average molecular weight (M?_w) was 9454. The dielectric α-relaxation of the oligomer was found to be governed by the Havriliak–Negami equation as well as the same series of oligomers with smaller M?_ws (388≦M?_w ≦ 3903). The dielectric relaxation times (τ)s for the oligomers with different M?_ws (1396 ≦ M?_w ≦ 9454) can be expressed by the Williams–Landel–Ferry (WLF) equation as a function of the glass transition temperature (T_g) at fixed temperatures from 70 to 100°C. The finding indicates that the T_g of the epoxide oligomer is calculated from the τ through the WLF equation, providing the relation between T_g and τ. The same type of WLF equation was also successfully applied to describe the T_g, dependence of the practical dielectric relaxation time (τ_p), which was obtained from the peak of the dielectric loss vs. frequency curve. The τ_p can be calculated more easily than the τ, based on the Havriliak–Negami equation, not only in the measurement of epoxide oligomer, but also in that of the reactive epoxy resin systems during curing. The T_g of an epoxy–aromatic amine system, which was determined from the τ_p nondestructively detected in the dielectric cure monitoring, was consistent with the T_g experimentally measured by differential scanning calorimetry (DSC).     

Comparaison des méthodes de fractionnement par chromatographie de perméation et par gradient d'elution pour la détermination des répartitions moléculaires des polypropylénes

Molecular weight distributions for polypropylene samples have been determined by a permeation fractionation method (GPC). Porous silica beads were used as a packing material for the columns. The set of columns allows a good separation of the polypropylene macromolecular chains in a range of molecular weights from 5000 to 1.5 × 10⁶, and the thermal and mechanical stabilities of these beads are very good. The calibration has been carried out with fractions of polypropylene of narrow molecular weight distribution prepared by a large-scale column fractionation. The molecular weights M?_w and M?_n and the ratios M?_w/M?_n calculated from the GPC curves show, in general, good agreement with the ones calculated from the column fractionation curves. However, the M?_w/M?_n ratios are always highter in the case of GPC fractionation. This could be due to diffusion phenomena.     

Melting and crystallization behavior of poly(tetrafluoroethylene). New method for molecular weight measurement of poly(tetrafluoroethylene) using a differential scanning calorimeter

Melting and crystallization behavior of virgin polytetrafluoroethylene have been studied using a differential scanning calorimeter. Following quantitative relationship was found between number average molecular weight of polytetrafluoroethylene and the heat of crystallization in the molecular weight range of 5.2 × 10⁵ to 4.5 × 10⁷: M?_n = 2.1 × 10¹⁰ ΔH_c^?5.16, where M?_n is number average molecular weight and ΔH_c is the heat of crystallization in cal/g. The heat of crystallization is independent of cooling rate ranging from 4 to 32°C/min. This relationship provides a simple rapid and reliable method for measuring the molecular weight of polytetrafluoroethylene.     

Thermosetting Polyetherimides: The Influence of Reactive Endgroup Type and Oligomer Molecular Weight on Synthesis,Network Formation,Adhesion Strength and Thermal Properties

The influence of the reactive endgroup on the synthesis, cure behavior and network properties of thermosetting polyetherimides was investigated. Reactive phenylethynyl, ethynyl and maleimide terminated etherimide oligomers were prepared and characterized. Optimal reaction conditions were established to produce fully endcapped oligomers with imidized structures and controlled molecular weight. The phenylethynyl and ethynyl endcapped systems were synthesized by ester-acid methods. The maleimide endcapped system utilized an amic-acid route. Phenylethynyl endcapped oligomers had good processibility and were thermally cured at high temperatures (350–380°C). The networks exhibited good thermal and hydrolytic stability and good adhesion strength, and are candidates for “primary'' bonding adhesives. In contrast, more reactive ethynyl and maleimide endcapped systems were prepared as “secondary'' bonding materials, which could be cured at temperatures lower than that of the T _g of the primary structure. Lap shear test results obtained from NMP-cast/methanol-extracted scrim-cloth-supported precursors confirmed that good adhesion to titanium at both room temperature and at 177°C was achieved when cured at 250°C-280°C. High glass transition temperatures and good thermal stability were achieved as determined by thermal analysis (DSC, TGA and DMA). Solvent extraction measurements confirmed that very high gel fractions were obtained, which is consistent with good chemical resistance.

The influence of molecular weight between crosslinks (Mc) on thermal and mechanical behavior was also investigated for 2,3,5,7 and 10k initial M _n values. Lower molecular weight oligomers exhibited lower T _g and cure temperatures, but higher cured network crosslink densities afforded higher T _g and higher gel fractions, but with reduced toughness.     

Effect of double bonds on the γ-radiation-induced crosslinking of polyethylene

Various low-density polyethylenes ranging in initial weight-average molecular weight (M?_w) from 7600 to 589000 having a ratio of M?_w to number-average molecular weight (M?_n) of about 5 were irradiated by γ-rays in vacuo at 30°C. Gel fractions were determined and analyzed by using the equation derived by Charlesby and Pinner. The following relationships were obtained when M?_w was used as the molecular weight: where r_g represents the gel point dosage (Mrad), [C?C]₀ is the sum of the initial contents of terminal vinyl and vinylidene unsaturations (mole/g polyethylene), and q₀ and p₀ are the probabilities of crosslinking and main-chain scission per monomer unit for a unit radiation dose in Mrad, respectively. Similar relationships to the equations described above were also obtained when M?_n was used. From the results, it was concluded that terminal vinyl and vinylidene unsaturations play an important role for the gel formation in the γ-radiation-induced crosslinking of polyethylene in vacuo at room temperature.     

1H magnetic relaxation times in isotactic polypropylene crystallized at high temperature. Effect of molecular weight

The temperature dependence of ¹H spin-lattice, T₁(H), and spin-spin, T₂(H), relaxation times of isotactic polypropylene with relatively low molecular weight, M?_w = 1.95 × 10⁵, and ultra-high molecular weight, M?_w = 1.78 × 10⁶, crystallized at high temperature (155° C) for a long time (200 h), was measured with a broad line pulse spectrometer. T₁(H) for the ultra-high molecular weight sample is shorter than that for the low molecular weight sample, while T₂(H) for the ultra-high molecular weight sample is longer than that for the low molecular weight sample. From the analysis of free induction decay (FID), three components of T₂(H), i.e. T_2c(H), T_2m(H) and T_2a(H), were obtained. These relaxation times are associated with the crystalline, intermediate and amorphous regions, respectively. Here the intermediate regions are the regions in which chain molecules have intermediate mobility between that of crystalline and amorphous regions. A decrease in signal intensity of methylene, methine and methyl carbons was measured as a function of delay time following a 90° pulse before cross-polarization. From the slope of a log (intensity) versus t² plot a relaxation time associated with rigid regions of proton nuclei, T_2r(H), was obtained, where the rigid regions are the regions in which cross-polarization occurs easily. The value of T_2r(H) lies between that of T_2c(H) and T_2m(H), indicating that the intermediate regions act as the rigid regions so far as cross-polarization is concerned.     

Engineering plastics from lignin. XV. Polyurethane films from chain-extended hydroxypropyl lignin

A series of polyurethane (PU) films was prepared from chain-extended hydroxypropyl lignins (CEHPL). In appearance, these films ranged from brittle and dark brown to rubbery and bronze. The thermal, mechanical, and network properties of these PUs were investigated by DMTA and DSC analysis. All films exhibited single T_g's which varied between ?53° and 101°C, depending on lignin content. From swelling experiments, molecular weight between crosslinks (M _c) was determined and found to vary over 2.5 orders of magnitude. The M _c's were related to the change in T_g that accompanied network formation. Stress–strain experiments showed a variation in Young's modulus between 7 and 1300 MPa. Most of the variation in material properties was related to lignin content and to a lesser extent to diisocyanate type, hexamethylene diisocyanate, or toluene diisocyanate. The source of the CEHPL had no effect on the observed properties. From these results it was concluded that the properties of PUs can be controlled and engineered for a wide variety of practical uses.     

Synthesis of new polyesters with chelating subunits by solution and interfacial polycondensation

New polyesters have been prepared by solution and interfacial polycondensation of acid chlorides obtained from bis‐α,ω‐polymethylene‐[(2‐methoxy)phenoxy]acetic acids, acid‐ether siderophores AE _n (n being the number of methylene units), and diols including bisphenol A, ethylene glycol and propanediol. For interfacial polycondensation, average molecular weights range from 1000 to 4500 g mol^?1; at higher temperature the molecular weights increase but the yield is smaller. Differential scanning calorimetry shows T_g ranging from ?5 to + 110 °C. Polyesters are crystalline with n = 6 and amorphous with n = 2 and 4; in addition, increasing n in AE _n increases molecular weights and T_g values. In solution polycondensation, the average molecular weights are around 3000 g mol^?1 and T_g is low. © 2001 Society of Chemical Industry     

Structure-property relationships in thermoplastic elastomers III. Segmented polyacetal-polyurethanes

Dihydroxy-terminated polyacetals had been synthesized from aldehydes and glycols and used as soft segments to obtain segmented polyurethane block copolymers. For soft segment ≥ 1700 M _n, the T_g ranges from–48 to ?58°C and is insensitive to the structures of diisocyanate and chain extender. The T_g of PacPU with 1350 M _n polyacetals is raised to ?38°C, and none was observed for shorter polyacetal chains. The copolymers can be synthesized to have a broad range of mechanical properties, such as modulus from 0.5 to 130 MPa, stress at break from 0.7 to 21 MPa, and elongation at break from 66 to 1300% through the variation of the constituents and composition. The rheologic properties are only slightly dependent on temperature for symmetrical diisocyanates but quite temperature sensitive with asymmetric diisocyanate copolymers. The polyacetals are selected to build in acid-catalyzed thermal decomposition of the thermoplastic elastomers. The extreme acid sensitivity of the polyacetal block is buffered in the coplymers.     

Molecular structure,crystallization behavior,and morphology of fractions obtained from an extrusion grade high-density polyethylene

An extrusion-grade of high density polyethylene (HOPE) (3 ethyl groups per 1000 carbons) has been divided into 16 fractions by preparative GPC and selective p-xylene extraction. The fractions, with molecular weights ranging from 900 to 1,000,000, have been studied by IR spectros-copy, DSC, WAXS, polarized microscopy, and small-angle light scattering (SALS), The average degree of chain branching (percent C₂H₅) is 0.5 percent for the part of the sample having a molecular weight lower than 10,000 and it decreases monotonically with increasing molecular weight, finally approaching 0.1 percent C2H5. A crystallinity depression with respect to linear PE equivalent to 20 percent/(percent C₂H₅) is recorded for all samples except for the very low molecular weight samples for which the crystallinity depression is much larger (30 to 35 percent/ (percent C₂H₅)). The unit cell volume increases with increasing percent C₂H₅, presumably due to the inclusion of ethyl groups in the crystals as interstitlals at 2gl kinks. The concentration of ethyl groups in the crystals (?_c) unanimously follows the relationship: ?_c(percent) = 0.32 + 0.25 log(percent C₂H₅) except for the low molecular weight fractions which have significantly lower values for ?_c. Our admittedly speculative explanation for this major discrepancy between high and low molecular weight samples is based on the idea that segments with ethyl groups close to chain ends have a greater difficulty in crystallizing than segments containing ethyl groups located at positions far from the chain ends. The fractions obtained from the extrusion-grade HDPE show a solidification temperature depression with respect to linear PE which can only be explained by the presence of chain branches in these samples. The depression is particularly pronounced for the low molecular weight samples as is expected from the data on molecular structure. Well-developed non-banded spherulites are observed in rapidly cooled (crystallized at about 35 K supercooling), low molecular weight samples (6,000 < M_w < 8,000)from the extrusion-grade HDPE in contrast to the axialites observed in linear PE of the same molecular weight and thermal treatment. This discrepancy in morphology has been related to the presence of ethyl groups in the extrusion grade HDPE fractions. Higher molecular weight samples (20,000 < M_w < 1,000,000)from the extrusion-grade HDPE and linear PE both display well-developed banded spherulites of similar nature as is expected due to the similarity in molecular structure of the two sets of sample.