Polyetherimide‐modified bismaleimide resins. II. Effect of polyetherimide content

A novel polyetherimide was prepared and used to improve the toughness of bismaleimide resin composed of bis(4‐maleimidediphenyl) methane and O,O′‐diallyl bisphenol A. The morphologies of the modified resins change from spherical particles to an inverted phase structure, depending on the modifier's content based on the scanning electronic microscopy results. Dynamic mechanical analysis is also used to characterize morphologies of the modified resins. The phase‐separation process of the modified system is traced by time‐resolved light scattering. The change in the light‐scattering profile with curing time showed that the phase separation mechanism depended on the modifier concentration. Phase separation took place via the spinodal decomposition mechanism in the PEI 15‐phr‐ and 20‐phr‐modified system. The fracture energy (G_IC) increased with PEI content in the modified system; in the PEI 15‐phr‐modified system, the G_IC value was three times greater than that of the unmodified BMI resin. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 350–358, 2001     

Rubber-modified epoxies. II. Influence of the cure schedule and rubber concentration on the generated morphology

The morphology of a system consisting of a bisphenol A diglycidylether (DGEBA) based epoxy, cured with a cycloaliphatic diamine (4,4′-diamino-3,3′-dimethyldicyclohexylmethane, 3DCM), in the presence of an epoxy-terminated butadience-acrylonitrile random copolymer (ETBN), was studied as a function of the cure schedule and the initial rubber concentration. Scanning (SEM) and transmission (TEM) electron microscopy, differential scanning calorimetry (DSC) and dynamic mechanical analysis were used to characterize the generated morphology. SEM results were not affected by the type of mechanical test and strain rate. Trends observed for the particle size distribution, the volume fraction of dispersed phase, the concentration of dispersed phase particles and the composition of both phases as a function of polymerization temperature and rubber concentration, were discussed. A correlation between the viscosity at the cloud point and the average size of dispersed phase particles was found for different systems, independently of the cure temperature and the initial rubber amount.     

Phase separation in rubber-modified thermoset resins: Optical microscopy and laser light scattering

Phase Separation of a prereacted rubber in di-, tri-, and tetrafunctional epoxy systems is discussed. Optical microscopy and laser light scattering are used to characterize the morphology and dimensios of the rubbery domains. Systems without rubber are transparent while systems containing 2%, 4%, 6%, and 8% and rubber exhibit distinct phase separation. The average domain size is about 1 μm In the 8% systems the particles are larger and in the trifunctional system the rubbery domains are in the shape of shell and core. Laser light scattering proves that the scattering particles are in the core observed in the optical micrographs. The difference in morphologies is explained by the difference in the chemical structure of the epoxy resin and the rubber content. Thermodynamic considerations and solubility parameters show that the di- and tetrafunctional systems have similar enthalpies of mixing while the trifunctional system is much more incompatible with the rubber.     

Thermoplastic vulcanizates obtained by reaction-induced phase separation: Interplay between phase separation dynamics,final morphology and mechanical properties

Reaction-induced phase separation (RIPS) of miscible blends of poly(?-caprolactone) (PCL) and an epoxy resin based on poly(propylene oxide) (PPO) was used to prepare thermoplastic vulcanizates (TPVs) with fine rubber dispersions. Scanning electron microscopy (SEM) confirmed the formation of cross-linked rubber particles dispersed in the thermoplastic matrix at PCL contents ≥20 wt%. The morphology development during phase separation was studied by optical microscopy (OM) and time-resolved small-angle light scattering (SALS). It was shown that higher curing temperatures lead to a decrease in rubber particle size, but at the same time lead to an increase in the extent of particle connectivity. In some cases, gelation of the PPO-rich phase limits full structure development, which leads to extensive connectivity between the dispersed rubber particles and a strong deterioration in tensile properties.     

Synthesis of polystyrene encapsulated nanosaponite composite latex via miniemulsion polymerization

The synthesis and characterization of polystyrene encapsulated nanosaponite composite suspension via miniemulsion polymerization are reported in this study. The particle size of nanoclay and its pre-modification are critical to successfully producing a stable complex suspension. The final products were characterized by X-ray diffraction spectra, transmission electron microscopy (TEM), scanning electron microscopy (SEM), thin window energy dispersive spectroscopy (EDS), and light scattering. The results show that ar-vinylbenzyltrimethylammonium chloride (VBTAC) modified nanosaponite could be fully exfoliated and encapsulated inside the polystyrene latex via in situ miniemulsion polymerization. When the concentration of hexadecane (a co-stabilizer used in the miniemulsion polymerization) was high, the final composite particles are composed mainly of spherical particles with size less than 100 nm, and a small number of hemispherical or bowl-structured particles of size ∼100 nm to 1000 nm. The phase separation due to the existence of large amounts of hexadecane accounted for the formation of a variety of morphologies.     

Nanostructures and thermomechanical properties of epoxy thermosets containing reactive diblock copolymer

Polystyrene‐block‐poly(glycidyl methacrylate) reactive diblock copolymer (PS‐b‐PGMA) was synthesized via atom transfer radical polymerization (ATRP). The diblock copolymer was characterized using nuclear magnetic resonance (NMR) spectroscopy and gel permeation chromatography (GPC). The cured epoxy thermosets with 10–20 nm PS particles were prepared by blending the diblock copolymer with epoxy resin. The nanostructures were examined by means of transmission electronic microscopy (TEM) and small angle X‐ray scattering (SAXS). The formation of the nanostructures was caused by the reaction‐induced microphase separation mechanism. It is significant that the glass transition temperatures (T_gs) of these epoxy thermosets were increased by the addition of PS‐b‐PGMA reactive block copolymer as revealed by both differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Influence of attapulgites on cure‐reaction‐induced phase separation in epoxy/poly(ether sulfone) blends

The influence of attapulgites (ATTs) on cure‐reaction‐induced phase separation in diglycidyl ether of bisphenol A/poly(ether sulfone) (PES) blends has been studied with scanning electron microscopy (SEM), transmission electron microscopy (TEM), optical microscopy, time‐resolved light scattering, and dynamic mechanical analysis at different ATT and PES concentrations. The SEM results show that the incorporation of a small amount of ATT into the blends can change the final phase morphology markedly and affect the secondary phase separation. The TEM results show that ATT particles are pinned down by the interfacial tension at the phase interfaces, and this slows the interfacial motion. In addition, the incorporation of a small amount of ATT particles can improve the modulus because of the increased interfacial interaction of the PES‐rich and epoxy‐rich phases. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Morphology and mechanical properties of ABS blends prepared from emulsion‐polymerized PB‐g‐SAN impact modifier with AIBN as initiator

A series of PB‐g‐SAN impact modifiers (polybutadiene particles grafted by styrene and acrylonitrile) are synthesized by seed emulsion copolymerization initiated by oil‐soluble initiator, azobisiobutyronitrile (AIBN). The ABS blends are obtained by mixing SAN resin with PB‐g‐SAN impact modifiers. The mechanical behavior and the phase morphology of ABS blends are investigated. The graft degree (GD) and grafting efficiency (GE) are investigated, and the high GD shows that AIBN has a fine initiating ability in emulsion grafting of PB‐g‐SAN impact modifiers. The morphology of the rubber particles is observed by the transmission electron microscopy (TEM). The TEM photograph shows that the PB‐g‐SAN impact modifier initiated by AIBN is more likely to form subinclusion inside the rubber particles. The dynamic mechanical analysis on ABS blends shows that the subinclusion inside the rubber phase strongly influences the T_g, maximum tan δ, and the storage modulus of the rubber phase. The mechanical test indicates that the ABS blends, which have the small and uniform subinclusions dispersed in the rubber particles, have the maximum impact strength. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Polymer crystallization from the metastable melt: The formation mechanism of spherulites

One of the most popular crystalline morphologies is a spherulite. An evidence is reported that the spherulite is crystallized through a dense packing state of small particles appearing in a droplet, which is caused by the primary phase separation of the melt in the metastable region of a phase diagram proposed by Olmsted et al. [Olmsted PD, Poon WCK, McLeish TCB, Terrill NJ, Ryan AJ. Phys Rev Lett 1998; 81: 373]. According to this phase diagram, the crystallization from the metastable state causes the nucleation and growth (N & G) of nematic domains, here named droplets, in the isotropic matrix. As a next step, the secondary phase separation of spinodal decomposition (SD) type into smectic and amorphous domains occurs inside the droplet where entanglements are excluded from the smectic to the amorphous domain; then such an SD structure turns into a densely packing structure of many small particles owing to surface tension. At this final stage of the induction period a long period peak of small-angle X-ray scattering (SAXS), so-called SAXS before WAXS, appears, which may be due to the average distance between these small particles. Furthermore, it is considered that crystalline lamellae are formed by radial and azimuthal fusion of these small particles inside the droplet, resulting in a spherulite. Such a type of crystallization occurs most commonly when flexible polymers are crystallized under the usual conditions. This tentative concept of spherulitic growth, which is completely different from a theory by Keith and Padden [Keith HD, Padden FJ. J Appl Phys 1963; 34: 2409], would give a new insight into problems of spherulites.     

Preparation of PBAS core-shell structured polymer by seeded emulsion polymerization and investigation in AS resin toughness

The preparation of poly(n-butyl acrylate)/poly(acrylonitrile-co-styrene), i.e., poly(BA)/poly(AN-co-St) (PBAS) core-shell structured modifier with controlled particle size was reported, and the mechanical properties of AS/PBAS blends were investigated. The modifier was prepared at a solid content of 50 wt % by a two-stage sequential emulsion polymerization. Dynamic light scattering (DLS) was used to monitor the particle diameters and showed that the particles grew without significant secondary nucleation occurring. The morphology was confirmed by means of transmission electron microscopy (TEM). According to the research on mechanical properties of the AS/PBAS blends, a remarkable toughening effect of PBAS on AS resin was found. By means of scanning electron microscopy (SEM) observation, the toughening mechanism was proposed to be crazing caused by rubber particles and shear yielding of AS matrix. Uniform dispersion of rubber particles in AS matrix was attributed to the good compatibility between AS and PBAS modifier. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Morphology and mechanical properties of layered silicate reinforced natural and polyurethane rubber blends produced by latex compounding

Natural rubber (NR), polyurethane rubber (PUR), and NR/PUR‐based nanocomposites were produced from the related latices by adding a pristine synthetic layered silicate (LS; sodium fluorohectorite) in 10 parts per hundred parts rubber (phr). The dispersion of the LS latices in the composite was studied by X‐ray diffraction (XRD) and transmission electron microscopy (TEM). Further information on the rubber/LS interaction was received from Fourier transform infrared spectroscopy (FTIR) and dynamic mechanical thermal analysis (DMTA). Tensile and tear tests were used to characterize the performance of the rubber nanocomposites. It was found that LS is more compatible and thus better intercalated by PUR than by NR. Further, LS was preferably located in the PUR phase in the blends, which exhibited excellent mechanical properties despite the incompatibility between NR and PUR. Nano‐reinforcement was best reflected in stiffness‐ and strength‐related properties of the rubber composites. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 543–551, 2004     

Influence of small rubber particles on the environmental stress cracking of high impact polystyrene

This article exploits the influence of rubber particle size (RPS) and rubber crosslinking on environmental stress cracking resistance (ESCR) of high impact polystyrene (HIPS), with special interest on the influence of small rubber particles fraction. Three commercial HIPS of high ESCR were selected and four batches of HIPS were prepared in‐house, including samples based on high cis and very high viscosity polybutadiene (PB). Their morphologies were analyzed by low angle laser light scattering, optical microscopy, and transmission electron microscopy, and the samples were submitted to flexural ESCR tests with fatty agents. The ESCR to sunflower oil was found to increase with the reduction of the rubber particles fraction smaller than 1–2 micron. Results have also confirmed that an increase in RPS is the key parameter to promote ESCR, although there is limit for RPS to be effective on ESCR improvement. The reduction of small rubber particles fraction in HIPS was achieved by using a high cis PB, that promotes low grafting efficiency of polystyrene onto PB backbone because of the low content of 1,2 vinyl isomer. Besides the ESCR improvements, HIPS with high cis PB showed higher elastic modulus and impact resistance than HIPS containing medium cis PB, which is desired for thickness reduction in food packaging and refrigeration cabinets. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Preparation of cationic functional polymer latexes and measurement of involatile monomer conversion

A series of poly(styrene‐co‐methacryloxyethylhexadecyldimethyl ammonium bromide), P(St‐DMHB), cationic particles were prepared by emulsion polymerization using 2,2′‐azobis(2‐methylpropionamide) dihydrochloride as the initiator with different levels of DMHB as the cationic functional comonomer. ζ potential, particle size, and size distribution of the particles were determined. Large discrepancy was observed between the particle size from dynamic light scattering and that from transmission electron microscopy. Results showed that particle size and ζ potentials were closely dependant on DMHB level used. A flocculation process for the cationic latex was established, and a method to estimate DMHB conversion was proposed. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

The study of polystyrene blends: The relationship of phases and structure in blends of polystyrene with ethylene–propylene diene monomer rubber and its grafted copolymer

Pressed films of blends of polystyrene (PS) with ethylene–propylene diene monomer rubber (EPDM) or grafted copolymer of styrene (St) onto EPDM (EPDM-g-St) rubber were examined by small-angle X-ray scattering (SAXS), and scanning electron microscope (SEM). Small-angle X-ray scattering from the relation of phase was analyzed using Porod's Law and led to value of interface layer on blends. The thickness of interface layer (σ_b) had a maximum value at 50/50 (PS–EPDM-g-St) on blends. The radius of gyration of dispersed phase (domain) and correlation distances a_c in blends of PS–EPDM-g-St were calculated using the data of SAXS. The morphology and structure of blends were investigated by SEM. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 805–810, 1998     

Mechanisms of plastic deformation in biodegradable polylactide/poly(1,4‐cis‐isoprene) blends

Polylactide (PLA), a main representative of biodegradable and made from renewable resources polymers, is surprisingly brittle at ambient temperature. In this article it is investigated how to increase its toughness by a strategy called “rubber toughening” using poly(1,4‐cis‐isoprene), a major component of natural rubber, which is immiscible with PLA, could be well dispersed in PLA matrix and is biodegradable. Immiscible blends of PLA with poly(1,4‐cis‐isoprene) were prepared by melt blending and their properties were studied and optimized. Incorporation of as low as 5 wt % of rubber increased the strain at break of compression molded film during uniaxial drawing, and also improved its tensile impact strength by 80%. The complex mechanism of plastic deformation in the blends leading to improvement of ductility and toughness was revealed. The rubbery particles initiated crazing at the early stages of deformation, as evidenced by transmission and scanning electron microscopy and also by small angle X‐ray scattering. Crazing was immediately followed by cavitation inside rubber particles, which further promoted shear yielding of PLA. The sequence of those mechanisms was proven by microscopic investigation. All three elementary mechanisms acting in the sequence indicated are responsible for surprisingly efficient toughening of PLA by a major component of natural rubber. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Morphology, structure and properties of a poly(1-butene)/montmorillonite nanocomposite

A specifically formulated nanocomposite based on isotactic poly(1-butene) (PB) and montmorillonite was studied, by wide angle X-ray diffraction (WAXD), small angle X-ray scattering (SAXS), transmission electron microscopy (TEM) and polarized light optical microscopy, investigating the polymorphism of the polymer, and examining the interaction between PB and the silicate. Montmorillonite was found to disrupt the ordered morphology of the polymer, determining a dramatic increase in the rate of the II→I phase transition. Interaction between polymer and clay was studied by TEM and SAXS also under a quantitative point of view. A significant enhancement of physical-mechanical properties was observed, even though exfoliation did not occur, but just a slight intercalation and a reduction in the size of tactoids.     

Thermo-Responsive Ultrafiltration Block Copolymer Membranes Based on Polystyrene-block-poly(diethyl acrylamide)

Within the present work, a thermo-responsive ultrafiltration membrane is manufactured based on a polystyrene-block-poly(diethyl acrylamide) block copolymer (BCP). The poly(diethyl acrylamide) block segment features a lower critical solution temperature (LCST) in water, similar to the well-known poly(N-isopropylacrylamide), but having increased biocompatibility and without exhibiting a hysteresis of the thermally induced switching behavior. The BCP is synthesized via sequential “living” anionic polymerization protocols and analyzed by ¹H-NMR spectroscopy, size exclusion chromatography, and differential scanning calorimetry. The resulting morphology in the bulk state is investigated by transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS) revealing the intended hexagonal cylindrical morphology. The BCPs form micelles in a binary mixture of tetrahydrofuran and dimethylformamide, where BCP composition and solvent affinities are discussed in light of the expected structure of these micelles and the resulting BCP membrane formation. The membranes are manufactured using the non-solvent induced phase separation (NIPS) process and are characterized via scanning electron microscopy (SEM) and water permeation measurements. The latter are carried out at room temperature and at 50 °C revealing up to a 23-fold increase of the permeance, when crossing the LCST of the poly(diethyl acrylamide) block segment in water.     

Interface morphology and phase separation in polymer-dispersed liquid crystal composites

It is widely appreciated that electro-optic activity in polymer-dispersed liquid crystals (PDLCs) depends on separation of the polymer and liquid crystal (LC) phases. Since the phase structure develops in a non-equilibrium system, the morphology of the LC domains depends on the details of the chemical and physical processes active during domain formation. The nature of the interface between the polymer and liquid crystal phases is of particular interest. This work discusses the two-phase morphology in an acrylate-based system that develops during polymerization-induced phase separation (PIPS). Using small-angle X-ray scattering (SAXS) and ultra-small-angle X-ray scattering (USAXS), we find that interfaces in PDLCs developed from an acrylate-based recipe are more disordered than generally appreciated. Information gained from SAXS and USAXS is compared to data from scanning electron microscopy (SEM) and transmission electron microscopy (TEM). To elucidate the apparent discrepancies between imaging and scattering, we investigated the effects of SEM sample preparation. We observe significant alteration of the interface morphology due to the leaching of the LC phase.     

Shrinkage and mechanical properties of unsaturated polyester reinforced with clay and core–shell rubber

Glassy unsaturated polyester (UP) resin was reinforced using an organically modified montmorillonite (OMMT) and toughened with core?Cshell rubber (CSR) particles. The nanostructure, morphology, and deformation mechanism of composite specimens were studied by small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM) and reflected optical microscopy (ROM). An intercalated nanostructure with partial exfoliation was observed in the UP reinforced by various amounts of OMMT. Locally clustered but globally good CSR particle dispersion in the UP matrix was evident in UP toughened with 5 and 10 wt% CSR particles. Simultaneous presence of OMMT and CSR particles in UP/OMMT/CSR hybrid composites was found to cause partial phase separation with bigger rubber particle agglomerates and lower clay-intergallery height increase. The effects of OMMT and CSR contents on volume shrinkage, impact fracture energy, fracture toughness, and compressive yield strength of UP were investigated. The introduction of OMMT of up to 3?wt% into the UP matrix lowered volume shrinkage to some extent, while further addition increased the shrinkage slightly. In the hybrid nanocomposites, the volume shrinkage decreased to a minimum level of 5.2?% with increases in OMMT level. The impact fracture energy of UP improved with increasing the OMMT level of up to 3?wt%, whereas its further addition decreased the impact fracture energy slightly due to the clay particle agglomeration. The hybrid composites with OMMT level below 3?wt% showed higher impact fracture energy compared to the reinforced UP specimens with the same OMMT levels. Interestingly, a synergism in the fracture toughness (K _IC) was observed in the hybrid composite containing 1?wt% OMMT and 10?wt% CSR particles. The presence of OMMT as reinforcement in the hybrid composites could compensate the lowering of the compressive yield strength caused by low-modulus CSR particles. The clay?Crubber particle interaction in the hybrid systems seems to increase the threshold of shear deformation of the UP matrix to some extent.     

Dynamic interplay between phase separation and crystallization in a poly(?-caprolactone)/poly(ethylene glycol) oligomer blend

We have investigated the crystallization effect on the phase separation of a poly(?-caprolactone) and poly(ethylene glycol) oligomer (PCL/PEGo) blending system using simultaneous small-angle light scattering and differential scanning calorimetry (SALS/DSC) as well as simultaneous small-angle X-ray scattering (SAXS), wide-angle X-ray scattering (WAXS), and DSC (SAXS/WAXS/DSC). When the PCL/PEGo system, of a weight ratio of 7/3, is quenched from a melt state (160 °C) to temperatures below the spinodal point and the melting temperature of PCL (63 °C), the structural evolution observed exhibits characteristics of (I) early stage of spinodal decomposition (SD), (II) transient pinning, (III) crystallization-induced depinning, and (IV) diffusion-limited crystallization. The time-dependent scattering data of SALS, SAXS and WAXS, covering a wide range of length scale, clearly show that the crystallization of PCL intervenes significantly in the ongoing viscoelastic phase separation of the system, only after the early stage of SD. The effect of preordering before crystallization revives the structural evolution pinned by the viscoelastic phase separation. The growth of SAXS intensity during the preordering period conforms to the Cahn-Hilliard theory. In the later stage of the phase separation, the PCL-rich matrix, of spherulite crystalline domains developed due to the faster crystallization kinetics, traps the isolated PEGo-rich domains of a slower viscoelastic separation.