Features of foaming of melts of mixtures of polyethylene and polypropylene

The “topological polymer chemistry” of amphiphilic linear and cyclic block copolymers at an air/water interface was investigated. A cyclic copolymer and two linear copolymers (AB-type diblock and ABA-type triblock copolymers) synthesized from the same monomers were used in this study. Relatively stable monolayers of these three copolymers were observed to form at an air/water interface. Similar condensed-phase temperature-dependent behaviors were observed in surface pressure–area isotherms for these three monolayers. Molecular orientations at the air/water interface for the two linear block copolymers were similar to that of the cyclic block copolymer. Atomic force microscopic observations of transferred films for the three polymer types revealed the formation of monolayers with very similar morphologies at the mesoscopic scale at room temperature and constant compression speed. ABA-type triblock linear copolymers adopted a fiber-like surface morphology via two-dimensional crystallization at low compression speeds. In contrast, the cyclic block copolymer formed a shapeless domain. Temperature-controlled out-of-plane X-ray diffraction (XRD) analysis of Langmuir–Blodgett (LB) films fabricated from both amphiphilic linear and cyclic block copolymers was performed to estimate the layer regularity at higher temperatures. Excellent heat-resistant properties of organized molecular films created from the cyclic copolymer were confirmed. Both copolymer types showed clear diffraction peaks at room temperature, indicating the formation of highly ordered layer structures. However, the layer structures of the linear copolymers gradually disordered when heated. Conversely, the regularity of cyclic copolymer LB multilayers did not change with heating up to 50 °C. Higher-order reflections (d002, d003) in the XRD patterns were also unchanged, indicative of a highly ordered structure. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015

The macromorphology of isotactic/atactic (iPP/aPP) and isotactic/syndiotactic (iPP/sPP) polypropylene mixtures is examined by optical microscopy. The spherulitic macrostructure of equimolecular weight [weight-average molecular weight (Mw) = 200k] iPP/aPP blends is volume-filling to very high aPP concentrations when the crystallization temperature is 130 °C. Similar spherulitic macrostructures (spherulite size and volume-filling nature) are observed for iPP homopolymer and a 50/50 iPP/aPP blend at low crystallization temperatures (115–135 °C). At higher crystallization temperatures (140–145 °C), a equimolecular weight (Mw = 200k) 50/50 iPP/aPP blend exhibits nodular texture that blurs the spherulitic boundaries. Double temperature jump experiments show that the nodular texture is due to melt phase separation that develops prior to crystallization. The upper critical solution temperature (UCST) of a 50/50 iPP/aPP blend (Mw = 200k) lies below 155 °C, and the blend is miscible at conventional melt processing temperatures. The UCST behavior is controlled by the blend molecular weight and aPP microstructure. aPP microstructures containing increased isospecific sequencing (although still noncrystalline) exhibit a reduced tendency for phase separation in 50/50 mixtures (Mw = 200k) and the absence of nodular texture at low undercoolings (140–145 °C). Equimolecular weight (Mw = 200k) 50/50 iPP/sPP mixtures exhibit phase-separated texture at all crystallization temperatures. The size scale of the phase-separated texture decreases with decreasing crystallization temperature because of a competition between crystallization and phase separation from a melt initially well mixed from the initial solution blending process. Extended melt annealing experiments show that the 50/50 iPP/sPP mixture (Mw = 200k) is immiscible in the melt at conventional melt processing temperatures. The iPP/sPP pair shows a much stronger tendency for phase separation than the iPP/aPP polymer pair. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1947–1964, 2000

The viscoelastic behavior and order‐disorder transition in mixtures of a block copolymer and a midblock‐associating resin were investigated. The block copolymers investigated were polystyrene‐block‐polysioprene‐block‐polystyrene (SIS) copolymers (Shell Development Company), specifically Kraton D‐1107, with the block molecular weights 10,000S‐120,000I‐10,000S, and Kraton D‐1111, with the block molecular weights 15,000S‐100,000I‐15,000S. The midblock‐associating resin investigated was a resin polymerized from C5 hydrocarbon, referred to as Piccotac 95BHT (Hercules Inc.), which is an aliphatic hydrocarbon containing considerable amounts of cyclic structures, with a weight‐average molecular weight of 1,100 and a glass transition temperature Tg of 43°C. In the investigation, mixtures of the block copolymer and Piccotac 95BHT were prepared with toluene as solvent. Temperature scans of the samples were made to obtain information on dynamic storage modulus G′, dynamic loss modulus G″, and loss tangent tan δ, using a Rheometrics dynamic mechanical spectrometer. It was found that Piccotac 95BHT decreased the plateau modulus G0N and increased the Tg of the polyisoprene midblock of the SIS block copolymer in the mixture. This experimental observation led to the conclusion that Piccotac 95BHT associates (or is compatible) with the rubbery polyisoprene midblock of the SIS block copolymer. The order‐disorder transition behavior of mixtures of SIS block copolymer and Piccotac 95BHT was also investigated by a rheological technique proposed by Han and Kim (Ref. 21). The order‐disorder transition temperature Tr (i.e., the temperature at which the ordered microdomain structure of the block copolymer completely disappears) of the SIS block copolymer decreased steadily with increasing amount of Piccotac 95BHT in the mixture. With the information determined on Tr, a phase diagram for the mixture was constructed, showing the boundary between the mesophase and homogeneous phase in the mixture. The phase diagram is in qualitative agreement with the theoretical predictions of Whitmore and Noolandi (Ref. 28).

The melting behavior of isotactic polypropylene was studied in the hot stage mounted on polarizing optical microscope supported by photomonitor. Over a wide range of crystallization temperature there are two main types of spherulites, α and β spherulites for this polymer α spherulites may exist in three forms α1, α2 and mixed of α1 and α2. The α2-form can be obtained at high crystallization temperature above 145°C and α1 can be observed at low crystallization temperatures below 132°C. The mixed α-spherulites can be obtained between 132°C and 145°C. However, the β-phase form small proportion of the total phase not more than 15% and usually exist when the crystallization temperature below 132°C. If the sample crystallized from the melt it shows usually two melting peaks. However, when the temperature of the sample was increased to crystallize in a second step at higher temperature it shows two melting peaks and if the second step of crystallization was very close to melting point it shows only one single peak, since the reorganization of crystals process will be stopped completely.

Poly[vinylidenefluoride‐co‐(tetrafluoroethylene)] (P(VDF‐TeFE)) exhibited clear spherulitic texture with negative birefringence. The number and growth rates of the spherulites decreased at high crystallization temperature than at low crystallization temperature. Nonetheless, overall larger spherulites were found at high crystallization temperature and the brightness of the spherulites increased very fast at low crystallization temperature, thereafter it seemed as diminution of birefringence. AFM was used to investigate the impact of organo modified nanodiamond(ND) on spherulitic textures, lamellar thickness, and thickness distribution of P(VDF‐TeFE) copolymer. Poly[ethylene‐co‐(tetrafluoroethylene)] (ETFE) also confirmed spherulites structure and the boundaries could be clearly observed. By incorporation of the organo modified nanodiamond (ND) and organo‐modified montmorillonite (MMT) in fluropolymer matrix, it was found that spherulitic texture was seriously disordered and their nanohybrids was found only to have poorly developed spherulite structure. Both of the nanohybrids samples show better crystallization temperature as compared to their neat copolymer samples. Furthermore, the incorporation of nanoparticles decreased the size of the spherulites. POLYM. ENG. SCI., 57:161–171, 2017. © 2016 Society of Plastics Engineers

The spectrum of PLLA is analyzed in order to investigate its crystallinity and crystalline morphologies. The carbonyl and ester bands of PLLA have been analyzed, and individual components have been successfully assigned. Nucleation always proceeds through curved lamellar crystals, this crystalline morphology being exclusive for low crystallization temperatures. At higher crystallization temperatures, a transition from curved crystals to flat lozenge-shaped lamellae is observed. Curved crystals with edge-on orientation and flat crystals with flat-on orientation affect the intensity of spectral bands. The total crystallinity has been obtained from a skeletal band at 955 cm-1. In addition, intensity changes observed in the CO stretching region during crystallization provide a simple procedure to obtain the relative population of the two crystalline morphologies. As crystallization temperature increases, the relative population of curved edge-on crystals is observed to decrease, but their population remains important even at the higher crystallization temperatures. The CO stretching region shows a complex profile that can be fully explained assuming intramolecular through bond coupling and factor group splitting. The latter is also affected by crystalline perfection; hence, the observed crystalline components strongly depend on the crystallization temperature. In the CO stretching region, perfectly flat crystals give two narrow components at 1767 and 1758 cm-1. Curved crystals obtained at low crystallization temperatures give a broader band located at 1760 cm-1 attributed to factor group splitting averaged over the different curvatures shown by this crystalline morphology. This contribution is expected to depend on crystallization temperature according to theoretical considerations (larger nuclei sizes). DSC melting shows a shoulder at lower temperature attributed to the presence of the less stable edge-on crystalline morphology. Finally, the ester C−O stretching region also shows factor group splitting in both the perpendicular (split ∼ 10 cm-1) and parallel (split ∼ 18 cm-1) components.

The viscoelastic behavior, order‐disorder transition, and phase equilibria in mixtures of a block copolymer and an endblock‐associating resin were investigated. The block copolymer was a polystyrene‐block‐polyisoprene‐block‐polystyrene (KRATON® D‐1107, Shell Development Co.) copolymer. The endblock‐associating resins investigated were two different grades of a commercially available random copolymer of poly(α‐methyl styrene) and polystyrene, one with a weight‐average molecular weight \[\bar M_{\rm w}\] of 710 (KRISTALEX® 3085, Hercules Inc.) and the other with \[\bar M_{\rm w}\] = 4100 (KRISTALEX® 5140, Hercules Inc.). Mixtures of various proportions of the block copolymer and the endblock‐associating resin were prepared in toluene solvent. With the mixtures, measurements of dynamic viscoelastic properties were made, namely, dynamic storage modulus G″ and dynamic loss modulus G″ as a function of temperature from temperature scans of the samples using a Rheometrics Mechanical Spectrometer. The following observations were made. (1) The plateau modulus of the block copolymer increased with increasing amount of KRISTALEX 3085 or KRISTALEX 5140, indicating that the low‐molecular‐weight resin was associated with the polystyrene microdomains of the block copolymer. (2) When KRISTALEX 3085 (up to 30 wt %) was added to the block copolymer, the glass transition temperature (Tg) of the polyisoprene midblock of the SIS block copolymer was shifted toward higher temperatures, indicating that part of the KRISTALEX 3085 added had associated with the rubbery midblock of the block copolymer. Also investigated was the order‐disorder transition behavior of the mixtures, using a rheological technique (log G′ versus log G″ plots) recently introduced by Han and Kim. It has been found that the order‐disorder transition temperature Tr of mixtures of the SIS block copolymer and KRISTALEX 3085 decreased steadily with increasing amount of KRISTALEX 3085, whereas the addition of KRISTALEX 5140 increased the Tr of the block copolymer. It was found by light scattering and hot‐stage microscopy that macrophase separation occurred in the KRATON 1107/KRISTALEX 5140 mixtures while microdomains of polystyrene were present in the block copolymer.

Miktoarm block copolymers (A2B2) composed of two poly(ε-caprolactone) (PCL) arms and two polystyrene arms (PS) were synthesized by a combination of ring-opening polymerization (ROP) and atom transfer radical polymerization (ATRP). Linear analogue PCL-b-PS diblock copolymer samples were also synthesized in almost identical composition regarding the content of each component. Almost all samples were found to be weakly segregated in the melt according to small angle X-ray scattering (SAXS) experiments. While the expected morphology (revealed by transmission electron microscopy, TEM) was found for the linear diblock copolymers, the miktoarm block copolymer samples exhibited different morphologies that indicated more entropic restrictions for chain stretching. For example, when a linear diblock copolymer with 41% PCL formed lamellae, the analogue miktoarm star copolymer with 39% PCL formed hexagonally packed PCL cylinders in a PS matrix. These results may imply changes in the phase diagram between miktoarm and linear block copolymers that have been previously predicted theoretically, however a larger number of samples should be used to corroborate this hypothesis. Additionally, the effect of polydispersity of the samples as a possible source of phase boundary variations should also be considered. The enhanced topological restrictions in the miktoarm star copolymers were also strongly reflected in the overall crystallization kinetics of the PCL component within the copolymers, as determined by differential scanning calorimetry (DSC). The supercooling needed for crystallization of the PCL component was much larger for the miktoarm star copolymers than for the linear analogue block copolymer samples of similar composition or even of similar morphology, while the crystallization rate was also depressed. The degree of crystallinity of the micro- or nanodomains was also a function of composition (decrease with PS content in the copolymers) or molecular architecture (lower in the stars than in linear copolymers) and as a general rule decreased as the level of confinement for the PCL component increased. Several kinetic theories of crystallization were applied to the overall isothermal crystallization data and regardless of the theory employed, the parameters proportional to the energy barriers for overall crystallization also increased with the confinement of the PCL component. Both the confinement degree and the influence of molecular architecture on the nucleation and crystallization of the PCL component generally increased with the content of covalently bonded glassy PS.

Biodegradable recombinant human erythropoietin loaded microspheres prepared from linear and star-branched block copolymers: Influence of encapsulation technique and polymer composition on particle characteristics

A comparative study of linear, Y-shaped and linear-dendritic methoxy poly(ethylene glycol)-block-polyamidoamine-block-poly(l-glutamic acid) block copolymers for doxorubicin delivery in vitro and in vivo

The shape of the dynamic moduli vs. temperature curves of polymers fundamentally depends on the temperature dependence of the relaxational properties of the polymers. When the frequency dependence of the reduced loss modulus is represented by an empirical function describing the dispersion peak, the distribution of the relaxation times and the storage modulus vs. frequency curve corresponding to that peak can be calculated. The temperature dependence of the reduced moduli can then be expressed as a function of the activation energy of the relaxation process and of a single parameter describing the width of the relaxational distribution function. This analysis is useful for a better understanding of the loss modulus vs. temperature spectrum and is applied to a linear polyethylene. Furthermore, a discussion is given of the differences in the spectra of linear polyethylenes and polypropylenes, mixtures of polyethylene and polypropylene, and copolymers of ethylene and propylene. When ethylene is randomly built into a polypropylene chain, the β‐transition of polypropylene is shifted to lower temperatures over a distance which is determined by the ethylene content. No shift is observed in mixtures of polyethylene and polypropylene. Here, the β‐peak of the loss modulus vs. temperature spectrum is only lowered and its height is a measure of the amount of amorphous polypropylene. The height of the loss modulus in the α‐region is determined by the total fraction of crystalline material in the mixture, the height of the γ‐peak is proportional to the ethylene content. The loss modulus spectrum can, to a large extent, be used for distinguishing between these different polymers.

The nonisothermal crystallization and morphology of three oxyethylene–oxybutylene block copolymers with different architectures (E50B70, B65E75B65, and E35B114E35) were compared with those of three blends (E56B27/B14, B37E77B37/B14, and E38B38E38/B14) with the same composition and morphology (E and B represent oxyethylene and oxybutylene units, respectively, and the subscripts denote the degree of polymerization), and the effect of the amorphous block was examined. The neat block copolymers had larger d-spacings and higher melting temperatures than the corresponding blends. In nonisothermal crystallization, the neat block copolymers had lower crystallization temperatures at high cooling rates. The difference in the crystallization temperatures became smaller at low cooling rates, and some of the neat block copolymers could have higher crystallization temperatures. Polarized optical microscopy showed that the neat block copolymers had smaller dimensions of crystal growth and smaller size of spherulites than the blends. The lower crystallization temperatures and less perfect morphology were attributed to the slower rate of conformational rearrangement of the amorphous block, which was required by the chain folding of the crystallizable block. This effect was more evident in the E35B114E35 triblock copolymer, in which both ends of the amorphous B block were immobilized at the interface. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 870–876, 2004

Dilatometric and calorimetric studies have been made of the fusion process of linear polyethylene crystallized by stirring xylene solutions at elevated temperatures. It is shown that the melting point of the crystals increases rapidly from 139.5°C to 145°C in the crystallization temperature range of 100–103°C and levels off to 146 ± 0.5°C, provided that very slow heating rates are employed. Stirrer‐crystallized samples treated with fuming nitric acid show higher crystalline contents. Comparison of their enthalpies of fusion and melting points indicate that higher molecular order along the fiber axis is associated with higher crystallization temperatures. This is in general agreement with corresponding results of other modes of crystallization. The attack of fuming nitric acid on stirrer crystals is characterized by weight‐loss curves similar to those of dilutesolution crystals and bulk polyethylene. The linear molecular weight dependence on time of exposure to nitric acid suggests that the oxidation proceeds mainly from the chain ends at a constant rate for samples stirred in the lower crystallization range, but an increased rate is observed for a sample stirred from xylene at 105°C. It is suggested that the lamellar overgrowths, most evident at low crystallization temperatures, are epitaxially attached to the fiber axis, whereas the smaller crossbandings observed at higher crystallization temperatures are possibly made up of elements of chains that are only partly incorporated in the highly ordered fibrous core.

Adult face flies, Musca autumnalis DeGeer, overwinter in reproductive diapause. In this study, we monitored temperature in a hibernaculum, and periodically assessed cold hardiness of flies from this site. Of flies collected in Keats, KS (39° 13′ N, 96° 42′ W) in late December and January, a40% exposed to −15°C for 5 h froze internally, whereas, the remainder successfully supercooled. Of flies collected in February, a70% froze internally. All flies that froze internally died. In December and January, all flies that did not freeze recovered fully and immediately, whereas in February nearly 80% of the flies that avoided freezing were killed or impaired. Unfed flies held at room temperature (23°C) for 8 h and then subjected to −15°C for 5 h had low crystallization temperatures, but were killed by chilling injury. Flies that were warmed, allowed food, and then held at 4°C overnight before being exposed to −15°C had higher crystallization temperatures than unfed flies. These data indicate that fully acclimatized face flies in midwinter are susceptible to freezing but not direct chilling injury, and that susceptibility to freezing and chilling injury in overwintering flies increases progressively with time. The laboratory studies show that, when warmed, diapause-induced and cold-acclimatized flies will take available food. Results from the laboratory studies suggest that flies warmed by exceptionally mild temperatures in midwinter are more susceptible to chilling injury from subsequent sudden cold weather, and that feeding when warmed increases crystallization temperatures. Higher crystallization temperatures could cause mortality under more mild conditions conditions that those in our test.

In the second study on melt‐miscible syndiotactic polystyrene (sPS) and poly(phenylene oxide) (PPO) blends, the effect of processing conditions on morphology, ultimate tensile properties, and the mode of fracture is reported. Bulk samples of the blends were molded and then crystallized from melt as well as from the quenched state at different temperatures. The spherulitic morphology of the melt‐crystallized blends, observed by scanning electron microscopy, revealed formation of complete, well‐developed spherulites whose texture increased in coarseness with increasing crystallization temperatures. In all the cold‐crystallized blends lamellar bundles formed a meshlike structure whose texture did not vary significantly with crystallization temperature. Depending on the crystallization temperature, 50/50 melt‐crystallized blends showed varying tensile properties and different modes of failure. In the samples with the largest amorphous domain size of 0.6 μm, the amorphous ellipsoids were cold drawn into fibrils during tensile loading and very high tensile strengths were recorded. The tensile properties for the other melt‐crystallized and all cold‐crystallized blends did not vary substantially with the changing crystallization temperature. The micrographs of the fractured surfaces of the melt‐crystallized blends suggested that, although intraspherulitic fracture occurred at low crystallization temperatures, interspherulitic fracture took place at high crystallization temperatures. The correlation of the morphology and mechanical properties suggests that melt‐miscible blends have good interfacial adhesion between phases and that, by varying composition and processing conditions, it might be possible to control amorphous domain sizes, which is critical in achieving better mechanical properties. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 87: 1984–1994, 2003