S. H. Carr

The effect of flow on the miscibility of a polymer blend

Results indicating the extent to which flow in a miscible polymer blend can displace the phase separation temperature are reported. Data were obtained on the system, polystyrene/poly (vinyl methyl ether), in the temperature ranges where it undergoes exsolution upon heating. The theoretical framework for the observed flow‐induced miscibility is described.

The development of spherulites from structural units in glassy poly[bisphenol-a-carbooate

Abstract Electron microscopic investigations have revealed that the surface of poly(bisphenol-A-carbonate) contains nodular units ca. 125 A in size. It is shown that annealing thin films at temperatures near the glass transition, Tg, causes these nodules to enlarge. Tensile deformation results in the nodules breaking apart as the film elongates; however, if the sample has been annealed prior to stretching, the nodules appear to rearrange as shear occurs between them. It is suggested that each nodule represents a region containing a near-crystalline degree of molecular order. Featherlike structures, many microns in extent, are also observed to form in thin films annealed near the glass transition. They are thought to be precursors of spherulites. By annealing slightly above the glass transition, the spherulite development sequence has been observed to involve the creation of lamellae from initially broad, radiating arms.

Self-compatibilization of polymer blends via novel solid-state shear extrusion pulverization

The mechanochemistry of a novel economical solid-state shear extrusion (SSSE) pulverization is investigated. SSSE compatibilizes incompatible blends in situ; the process has great potential in recycling of post-consumer plastic waste (PCPW). It is proposed that SSSE causes this self-compatibilization of blends by rupturing polymer chains and allowing them to recombine with their neighboring chains. When this recombination involves dissimilar chains at an interface between powder particles, block copolymers are formed, and if the chain transfer reactions are possible, graft copolymers are formed. These copolymers at the interfaces in the phase-separated, incompatible blend lower the interfacial tension and increase the adhesion at the interfaces, thus compatibilizing the blend. Our nuclear magnetic resonance (NMR) and rheology studies reveal the formation of long chain branches (LCBs) in an linear low-density polyethylene (LLDPE), which is equivalent to the formation of graft copolymers in blends. With NMR, an increase from ∼ 0.2 to ∼ 2.0 of the number of LCBs per 1000 carbon atoms is observed due to pulverization of the LLDPE.

Formation of bimodal crystal textures in polypropylene

Electron microscopy, selected area electron diffraction, X-ray diffraction, and dynamic-mechanical testing have been used to study flow-crystallized and hot drawn isotactic polypropylene. As a result of these investigations, it was found that bimodal crystal textures can apparently be formed by at least two different treatments, but the corresponding morphologies are completely different. Flow-induced crystallization was observed to result in a microstructure of lamellae oriented perpendicular to the flow direction, while hot drawing of polypropylene films above a critical temperature produced a morphology of microfibrils lying parallel to the draw direction. Below this critical temperature, drawing produced a fibrillar morphology having only a typical unimodal fibre texture. As a result of information obtained here, a mechanism involving epitaxial deposition of chain segments onto growing lamellae is concluded to be responsible for formation of the bimodal crystal texture in flow-crystallized material.

An analysis of flow-induced miscibility of polymer blends

Abstract The phase behavior of blends of polystyrene and poly(vinyl methyl ether) undergoing shearing flow was examined. Experiments conducted at a constant level of flow-induced strain found that such flow elevates the phase boundary and pro-motes miscibility. In the framework of the Cahn-Hilliard model for spinodal decomposition, an expression was developed which predicts the effect of an external flow field on the spinodal. The basis for this expression is the minimization of strain energy in a two-component polymer system that is in a thermodynamic state near its phase-separation point. The zero shear viscosities of the blends were measured and demonstrated to be an indication of one- or two-phase flow.

Influence of cure via network structure on mechanical properties of a free-radical polymerizing thermoset

Abstract An improved understanding has been achieved regarding the relationships among cure chemistry, network structure, and final physical properties of vinyl ester (VE) resins, a thermoset polymer often used as the matrix of fiber reinforced polymers. Mechanical properties of the polymer are found to depend on both cure schedule and cure formulation. The possibilities of phase separation and micro-gel formation being the cause for these differences in mechanical properties are examined. The VE/styrene (S) system does not phase separate under the conditions studied. Though bulk properties of the resin are unaffected by the details of the cure, the microscopic morphology, in particular the type of cross-link formed (intermolecular bond or intramolecular bond), is sensitive to both cure temperature and initiation mechanism as determined by cure formulation. An analysis of cure kinetics shows that both temperature and initiation mechanism affect the apparent ‘reaction order’ of the VE/S system as determined by the autocatalytic equation. This apparent reaction order is interpreted as being an indication of the degree of heterogeneity in the resin. By controlling cure temperature and cure formulation, it is possible to minimize the apparent reaction order and thereby optimize physical properties. Finally, a theory is adapted from other non-network polymer systems to qualitatively describe how cure temperature and initiation mechanism may alter the heterogeneity in network structure via micro-gel formation and how these changes in structure affect changes in the mechanical properties.

Morphology and deformation of melt-spun polyethylene fibers

Abstract Studies have been conducted on the melt-spinning and tensile properties of polyethylene fibers. The process whereby a filament of molten polyethylene is converted into a fiber has been noted to occur within an identifiable constriction zone. It has been inferred that this constriction zone results from a mechanical instability caused by different viscous compliances in fiber exterior and interior. This difference makes the skin move faster than the core and creates a flow field that produces a stress-crystallized morphology in the final melt-spun fibers. Birefringence studies on transverse thin sections suggest that the chains are tilted with respect to the fiber axis at an angle which varies with radius but is axially symmetric. Elastic strain in these fibers results largely from cooperative bending and bowing of lamellae. Plastic deformation beyond the yield point appears chiefly to be derived from lamellar tilting in combination with the formation of microfibrillar crystallites.

Trace levels of mechanochemical effects in pulverized polyolefins

This research investigated the structural changes that occur on different polyethylene polymer systems as a result of a novel pulverization process called solid-state shear pulverization (S3P). High-density polyethylene, low-density polyethylene, and two forms of linear low-density polyethylene were run through a pulverizer under high shear conditions as well as low shear conditions. The physical properties were examined before and after the pulverization via melt index, melt rheology, GPC, and DSC, techniques. The low shear pulverization did not noticeably alter the physical properties of the polymers. In contrast, high shear pulverization did result in an increase in viscosity as observed by melt index and oscillatory shear experiments, although solid-state and bulk properties as observed by DSC and GPC were not affected. These results indicate that a small amount of mechanochemically induced changes occur as a result of the pulverization process, including incorporation of a small amount of long-chain branches randomly placed on a few of the polymer chains. No evidence of short-chain branching resulting from S3P processing was found in these systems.

Electric field-induced structure in poly(acrylonitrile)

SummaryExperimental evidence has been obtained for alteration of the molecular organization in polyacrylonitrile which has been given a thermoelectric treatment that creates a persistent electrical polarization. These studies have involved infrared spectroscopy and X-ray diffractometry, and their results have been interpreted as suggesting the existence of dipolar clusters in polarized material. These clusters cause polyacrylonitrile chains to adopt a structure which is different from any previously reported for this material. It is suggested that stability of the polarized state of polyacrylonitrile results, in part, from this new molecular organization.

The J integral fracture toughness and damage zone morphology in polyethylenes

Abstract The J integral analysis of compact tension samples has been used to evaluate plane strain fracture toughness of various polyethylenes at room temperature. Crack propagation commences from a razor notch in high-density polyethylene at J Ic = 1.7 kJ m −2 . Plastic deformation is confined to a small craze region (about 300 μm long, 20 μm high) through which the crack subsequently propagates. A tough copolymer of PE3408 material resists crack advance until J Ic = 8.2 kJ m −2 . Here again crack propagation is through a craze, though craze length exceeds 1 mm. Toughness is also imparted by the formation of shear bands near the notch tip. Low-density polyethylene does not really fracture under the present test conditions; this material responds by general yielding and blunting of the notch tip.