Fringed Micellar Crystals Research Articles

Reaction: Size Distribution in Olefin Block Copolymers: Not Bad at All

Hongyu Chen completed his PhD in macromolecular science at Case Western Reserve University in 2010. He is a fellow of the Dow Chemical Company, where he focuses on developing new products. He is also a fellow of the American Physical Society and American Chemical Society. Valeriy Ginzburg completed his PhD in polymer physics at the Moscow Institute of Physics and Technology in 1992. He is a senior scientist at the Dow Chemical Company and a fellow of the American Physical Society.

Blends of ethylene–octene copolymers with different chain architectures – Morphology, thermal and mechanical behavior

Blends of two elastomeric ethylene–octene copolymers with similar octene contents having a random (ORC) and a blocky architecture (OBC) are prepared by melt mixing. The thermal and mechanical properties of ORC, OBC and their blends are investigated by DSC, dynamic mechanical analysis and tensile tests. The morphology of the semi-crystalline samples is studied by AFM and WAXS. Two types of crystals have been observed: (i) Orthorhombic crystals forming lamellae with an estimated thickness of about 13 nm composed mainly of long polyethylene-like sequences of OBC that melt a temperature of about 120 °C and (ii) fringed micellar crystals with a thickness of 2–4 nm formed basically by short polyethylene-like sequences of ORC that have melting temperatures between 30 and 80 °C. The amorphous phase contains a relatively homogeneous mixture of segments of both components indicated by the relatively uniform shape of the loss modulus peaks from dymamic-mechanical measurements for all investigated copolymers and blends. ORC crystallization is hindered in blends as indicated by lower melting enthalpies. This might be related to the high octene content of the amorphous phase at the relevant crystallization temperature as well as geometrical constraints since ORC crystallization occurs in an already semi-crystalline polymer. The results of tensile tests show that the mechanical behavior can be tailored via blend composition and morphology of the semi-crystalline material. The findings clearly indicate that blending is a powerful strategy to optimize the properties of polyolefin-based copolymers.

Photodegradation of ethylene–octene copolymers with different octene contents

The influence of the octene content on the photodegradation behaviour of ethylene–octene copolymers (EOCs) was revealed by investigating the photooxidation of low density polyethylene (LDPE) and EOCs with different octene contents through a series of characterisation methods. LDPE was very sensitive to ultraviolet light and the photostabilities of EOCs decreased with increasing octene concentration. The photodegradation of all samples produced hydroxyl, carbonyl and vinyl groups. The ease of chain crosslinking and scission was increased as the octene content rose. Crosslinking predominated in late irradiation period of LDPE while chain scission was dominant in that of EOCs. Annealing and chain scission promoted the secondary crystallisation of the crystallisable chain segments. Chain scission enhanced the crystallisation ability of the irradiated EOCs while it decreased that of the weathered LDPE. The photostabilities of crystals could be ranked as follows: the chain-folded lamellar crystals > the bundled crystals > the fringed micellar crystals. The thermal stabilities and mechanical properties of samples decreased with increasing irradiation time and the decreasing extent was correlated with the comonomer content.

Deformation of elastomeric polyolefin spherulites

AbstractThe ethylene‐octene block copolymers in this study consist of long crystallizable sequences with low comonomer content alternating with rubbery amorphous blocks with high comonomer content. The crystallizable blocks form lamellae that organize into space‐filling spherulites even when the fraction of crystallizable block is so low that the crystallinity is only 7%. These unusual spherulites are highly elastic and recover from strains as high as 300%. This new class of thermoplastic elastomers is fundamentally different from conventional elastomeric olefin copolymers that depend on isolated, fringed micellar‐like crystals to provide the junctions for the elastomeric network. The elastomeric block copolymers are shown to be unique in that a hierarchical organization of space‐filling lamellar spherulites provides the junctions for the elastomeric network. The deformation of the elastic spherulites is readily studied with small angle light scattering, wide angle X‐ray diffractograms, and atomic force microscopy. At strains in excess of 300%, the spherulites break up into a fibrillar structure following lamellar deformation processes that are similar to those established for high density ethylenic polymers. The crystalline transformation produces a stiffer elastomer that exhibits complete recovery on subsequent loadings. Similar experiments on elastomeric random ethylene‐octene copolymers where fringed micellar crystals provide the physical crosslinks that connect the rubbery, amorphous chain segments reveal significant differences. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1313–1330, 2009

Comparing elastomeric behavior of block and random ethylene–octene copolymers

AbstractThis work compared the elastomeric properties of two low‐crystallinity ethylene–octene copolymers. One was a block copolymer with lamellar crystals and the other was a random copolymer with fringed micellar crystals. The comparison of the stress–strain behavior at 23°C revealed that the initial elastic modulus and the yield stress depended only on the crystallinity of the copolymer. When the temperature was raised above 23°C, melting of the fringed micellar crystals of the random copolymer caused a rapid decrease in the modulus. Some decrease in the modulus of the block copolymer over the same temperature range was attributed to the crystalline α‐relaxation. Both polymers exhibited strain‐hardening, ultimate fracture at high strains, and high recovery after fracture. However, in the block copolymer, the onset of strain‐hardening and the ultimate fracture occurred at higher strains. The block copolymer also showed higher recovery from high strains. The initial stretching resulted in a permanent change in the stress–strain curve. It was suggested that following the onset of crystal slippage at the yield, the crystals underwent permanent structural changes through the course of the strain‐hardening region. The transformation of the fringed micellar crystals occurred at lower strains than the transformation of the lamellar crystals. The extent of the structural transformation was described by the crosslink density and the strain‐hardening coefficient extracted from elasticity theory. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Adhesion of statistical and blocky ethylene–octene copolymers to polypropylene

This study examined the effect of chain microstructure on the adhesion of ethylene–octene copolymers to polypropylene. The copolymers were candidates for compatibilization of polypropylene (PP) and high density polyethylene (HDPE) blends, and included a blocky copolymer, a statistical copolymer that had the same composition as the soft segment of the blocky copolymer, and a statistical copolymer that had the same comonomer content and crystallinity as the blocky copolymer. The compatibilized melt blend was modeled by a microlayered, one-dimensional structure consisting of alternating layers of PP and HDPE, each separated by a thin tie-layer. The microlayered structure made it possible to directly measure the adhesion using the T-peel test. Infrared analysis of matching peel fracture surfaces established that fracture occurred adhesively at the interface between PP and the tie-layer. Direct observation of the damage zone at the crack tip and microscopic examination of the fracture surfaces revealed that the tie-layer was highly deformed before final separation occurred at the interface. The substantially higher delamination toughness of the blocky copolymer compared to the statistical copolymers could be accounted for by considering both interspherulitic mechanically interlocking influxes and intraspherulitic entrapment of interdiffused tie-layer chains. The blocky copolymer also retained delamination toughness to a higher temperature due to the greater stability of lamellar crystals compared to fringed micellar crystals.

Development of lamellar morphology in poly(ethylene terephthalate)/polycarbonate blends

We have studied the lamellar-level morphology of poly(ethylene terephthalate) (PET)/polycarbonate (PC) blends using small-angle X-ray scattering (SAXS). Measurements were made as a function of the holding time in the melt. We determined the morphological parameters at the lamellar level by correlation function analysis of the SAXS data. An increased amorphous layer thickness was identified in the blend, indicating that some PC was incorporated into the interlamellar regions of PET during crystallization. The blend also exhibits a larger lamellar crystalline thickness (l c ) than that of pure PET. A possible reason for the increase inl c is that the inclusion of the PC molecules in the interlamellar regions causes an increase in the surface free energy of folding. At the early stage of isothermal crystallization, we observed a rapid drop in the value ofl c in the blend; this finding indicates that a relatively large fraction of secondary crystals form during the primary crystallization. In contrast, the value ofl c for the sample that underwent a prolonged holding time increased with time in the secondary crystallization-dominant regime; this observation suggests that the disruption of chain periodicity, which results from transesterification between the two polymers, favors the development of fringed micellar crystals that have larger values ofl c , rather than the development of normal chain-folded crystals.

Characterization of polyethylene with partially random chlorine substitution

AbstractA series of chlorine‐containing polymers were prepared by ring‐opening metathesis polymerization (ROMP) followed by hydrogenation. This synthesis route was chosen specifically so that chain microstructures would be obtained that resembled copolymers of ethylene and vinyl chloride. The chlorine content was varied by the copolymerization of 5‐chlorocyclooctene and cyclooctene. Differential scanning calorimetry, light microscopy, tapping‐mode atomic force microscopy, wide‐angle X‐ray diffraction (WAXD), and density were employed to characterize the polymers. The copolymers had certain restrictions on the length of the methylene sequence between substituted carbons, however, ROMP copolymerization introduced enough variation in the methylene sequence length that model copolymers with the equivalent of 14 mol % vinyl chloride or less closely resembled random copolymers of ethylene and vinyl chloride. These materials organized as spherulites and exhibited the orthorhombic crystal form. Constraints on the placement of chlorine atoms strongly affected the crystallization of polymers with more than the equivalent of 14 mol % vinyl chloride. More regular chlorine substitution along the polyethylene chain translated into better ordered crystal structures with sharp melting peaks. The granular morphology of these materials at ambient temperature was interpreted as fringed micellar crystals. The WAXD patterns provided definitive evidence that chains in the fringed micelle took the hexagonal crystal form. The lower density hexagonal form facilitated the crystallization of short ethylene sequences and accommodated chlorine atoms more easily than the orthorhombic form. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2062–2070, 2003

Relationship of deformation behavior to thermal transitions of ethylene/styrene and ethylene/octene copolymers

AbstractThe stress‐strain response of low‐crystallinity ethylene‐octene (EO) and ethylene‐styrene (ES) copolymers with 7–20 mol % comonomer was compared over a temperature range that spanned the glass‐transition and crystal melting regions. Above the onset temperature of the glass transition, the copolymers exhibited elastomeric behavior with low initial modulus, uniform deformation to high strains, and high recovery after the stress was released. In the glass‐transition range, an initial low‐stress elastomeric response was followed by a distinct “bump” in the stress‐strain curve. On the basis of the temperature and rate dependence of the stress‐strain curve, local strain‐rate measurements, local temperature changes, and recovery characteristics, the “bump” was identified as high strain yielding. Hence, the stress‐strain curve sequentially exhibited the features of elastomeric and plastic deformation. Following high strain yielding, strain hardening dramatically increased the fracture strength. This behavior was defined as elastomeric‐plastic. Elastomeric‐plastic behavior in the broad glass‐transition range constituted a gradual transition from elastomeric behavior at higher temperatures to low‐temperature plastic behavior with high modulus and macroscopic necking. Because of the lower glass‐transition temperature of EO, −40 °C as compared with −10 °C for ES, the onset of elastomeric‐plastic behavior occurred at a significantly lower temperature. The concept of a network of flexible chains with fringed micellar crystals serving as the multifunctional junctions that provides the structural basis for elastomeric behavior of low‐crystallinity ethylene copolymers was extended to elastomeric‐plastic behavior by considering a network with a fraction of rigid, glassy chains. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 40: 142–152, 2002

Classification of ethylene-styrene interpolymers based on comonomer content

Copolymerization of ethylene and styrene by the INSITETM technology from Dow presents a new polymer family identified as ethylene–styrene interpolymers (ESI). Based on the combined observations from melting behavior, density, dynamic mechanical response, and tensile deformation, a classification scheme with 3 distinct categories is proposed. Polymers with up to 50 wt % styrene are semicrystalline and are classified as type E. The stress–strain behavior of low-crystallinity polymers at ambient temperature exhibits elastomeric characteristics with low initial modulus, a gradual increase in the slope of the stress–strain curve at higher strains, and large instantaneous recovery. The structural origin of the elastomeric behavior is probably a network of flexible chains with fringed micellar crystals serving as multifunctional junctions. Polymers with more than 50 wt % styrene are amorphous. Because the range of glass transition temperatures encompasses ambient temperature (nominally 25°C), it is useful to differentiate ESIs that are above the glass transition as type M and those that are below the glass transition as type S. Type M polymers behave as rubber-like liquids. They have the lowest modulus and lowest stress levels. Some elastic characteristics are attributed to the entanglement network. Type S polymers exhibit large strain rate sensitivity with glassy behavior at short times and rubbery behavior at longer times. The term “glasstomer” is coined to describe these polymers. The division between type M and type S is based on chain dynamics, rather than solid state structure, and thus depends on the temperature of interest. At ambient temperature, ESIs with 50 to 70 wt % styrene are classified as type M; polymers with more than 70 wt % styrene are classified as type S. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 109–119, 1998

Deformation of Elastomeric Ethylene−Octene Copolymers

The elastomeric behavior of low-crystallinity ethylene−octene copolymers prepared by Dow's INSITE constrained geometry catalyst technology is described. Deformation in uniaxial tension was examined as a function of comonomer content and molecular weight. Within the melting range of copolymers, temperature was used as an experimental variable to reveal the relationship between crystallinity and stress response. The concept of a network of flexible chains with fringed micellar crystals serving as the multifunctional junctions provided the structural basis for analysis of the elastic behavior. The rubber modulus scaled with crystallinity. Furthermore, the dimension of the fringed micellar junction obtained from the modulus correlated well with the average crystallizable sequence length of the copolymer. Because classical rubber theory could not account for the large strain dependence of the modulus, a theory which incorporates the contribution of entanglements to the network response was considered. Slip-lin...

Study of crystal texture in pvc gels by x‐ray diffraction and infrared dichroism

AbstractAn enquiry is presented into the nature and origin of thermoreversible gelation of poly(vinyl chloride) (PVC). In line with a previous paper it is demonstrated by direct x‐ray diffraction evidence that the origin of the gelation is crystallization, and that this crystallinity can be conspicuously apparent even for the usual technological polymer. In addition some unusual diffraction effects become apparent such as do not obviously follow from existing notions of the PVC structure. Combined infrared dichroism and x‐ray studies revealed that these new effects are attributable to a two‐component nature of the crystallinity where the components are distinguished by their orientation behavior, lateral extension, and possibly morphology of the crystallites. Some of the crystals orient with their a axes (type A crystals) and some with their c axes (type B crystals) along the stretch direction. Other more complex orientability differences are also manifest during film formation. Crystals B are smaller, and are believed to correspond to the network forming junctions, hence to fringed micellar crystals. Crystals A have a less evident connection with the network and are hypothesized to be of lamellar character, the behavior of which is closely simulated by lamellar crystal additives (stabilizer crystals). High‐temperature x‐ray work revealed no melting of crystals even in the temperature range where endotherms appear in the thermograms.

Gelation properties of partial benzoylated isotactic polystrene and its implications to the gel structure of isotactic polystyrene

Isotactic polystyrene (IPS) solution crystals have been benzoylated in heterogeneous medium. By this method, a multi-block copolymer was obtained. The discussed benzoylated IPS crystals (BzIPS) show similar gelation properties as pure isotactic polystyrene. It is proved that the crystalline cross-links in the gel of the BzIPS polymers are formed by the IPS segments which formed the crystalline core in the original IPS solution crystals. These fringed micellar crystals do not show the familiar 3 1 helical structure of IPS. The proposal is made that at low temperature IPS as well as BzIPS can crystallize in two different crystal structures. A suggestion is made that the formation of the new structure of IPS may be the origin of some anomalies in BzIPS and IPS crystallization at large undercoolings e.g. crystallization rate and annealing behaviour.