丙烯超临界聚合中原生晶特征

通过广角X射线衍射(WAXD)对由丙烯超临界聚合和传统本体淤浆法制得的等规聚丙烯原生结晶特征进行了研究,并讨论了聚合工艺、预聚温度、粒径、分子量等对原生晶特征的影响。结果表明,超临界聚合法制备的等规聚丙烯(SC-iPP)呈现出α和γ的混合晶型,而采用传统淤浆法制备的等规聚丙烯(C-iPP)只含有单一α晶型,这是因为超临界的高温高压状态,使得体系传热传质阻力较小,更有利于原生晶型的发展。此外,预聚温度、粒径、分子量等因素也对丙烯超临界聚合所制得的SC-iPP结晶特征有显著影响。     

Foaming of linear isotactic polypropylene based on its non-isothermal crystallization behaviors under compressed CO2

The non-isothermal crystallization behaviors of isotactic polypropylene (iPP) under ambient N₂ and compressed CO₂ (5–50 bar) at cooling rates of 0.2–5.0 °C/min were carefully studied using high-pressure differential scanning calorimeter. The presence of compressed CO₂ had strong plasticization effect on the iPP matrix and retarded the formation of critical size nuclei, which effectively postponed the crystallization peak to lower temperature region. On the basis of these findings, a new foaming strategy was utilized to fabricate iPP foams using the ordinary unmodified linear iPP with supercritical CO₂ as the foaming agent. The foaming temperature range of this strategy was determined to be as wide as 40 °C and the upper and lower temperature limits were 155 and 105 °C, which were determined by the melt strength and crystallization temperature of the iPP specimen under supercritical CO₂, respectively. Due to the acute depression of CO₂ solubility in the iPP matrix during the foaming process, the iPP foams with the bi-modal cell structure were fabricated.     

Deformation behavior of isotactic polypropylene crystallized via a mesophase

The mechanical behavior of semicrystalline isotactic polypropylene (iPP) of different crystallinity, crystal morphology and superstructure was investigated by standard tensile stress–strain analysis, dynamic-mechanical analysis, and in situ observation of the deformation by atomic force microscopy (AFM). Emphasis is put on the comparison of the mechanical characteristics of specimens containing either non-isometric lamellae, being arranged in spherulites, or nodular isometric domains, which are not organized in a superstructure. The formation of lamellae/spherulites and of nodules was controlled by the conditions of crystallization. The replacement of cross-hatched monoclinic lamellae and a spherulitic superstructure by randomly arranged isometric nodules leads to a distinct increase of the ductility and toughness, even if the crystallinity is identical. The modulus of elasticity and the yield stress increase as expected with increasing crystallinity. Slightly lower values of Young’s modulus and yield strength are detected if samples contained non-lamellar crystals in a non-spherulitic superstructure, proving an effect of the crystal shape on the deformation behavior. For the first time, tensile deformation of semicrystalline iPP which contains nodular ordered domains instead of lamellae has been followed by in situ AFM.     

The morphology of irradiated isotactic polypropylene

The research in this article explores the response of semicrystalline isotactic polypropylene to gamma radiation in air, and relates the morphological changes of the polymer to corresponding changes in mechanical properties. The effect of the initial morphology of the polymer on its response to irradiation is considered using infrared spectroscopy (FTIR), small‐ and wide‐angle X‐ray scattering, dynamic mechanical thermal analysis (DMTA), and mechanical testing. The extent of chain scission and crosslinking is dependent on the dose but not the initial starting morphology. These chemical changes cause the crystallinity to increase slightly, and the glass transition temperature to rise by a few degrees in all samples, but the overall morphology is only subtly changed. In contrast, a major deterioration in mechanical properties is caused. The effects of the irradiation observed under these conditions are similar in each material and the ultimate properties determined by the properties seen in the original material. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 2234–2242, 1999     

Crystallization behavior and crystallization kinetic studies of isotactic polypropylene modified by long‐chain branching polypropylene

In this article, we discuss the crystallization behavior and crystallization kinetics of isotactic polypropylene (iPP) modified by long‐chain‐branching (LCB) high‐melt‐strength iPP over a wide composition range, that is, LCB‐iPP from 10 to 50 wt %. Over the entire range we investigated, the presence of LCB‐iPP accelerated crystallization in both the isothermal crystallization process and nonisothermal crystallization process, even when the LCB‐iPP content was as low as 10%, and both crystallization processes were enhanced more significantly as the LCB‐iPP content increased. Hoffman–Lauritzen theory analysis revealed that the fold‐free energy decreased effectively with the occurrence of the LCB structure, although the growth rate of spherulites was depressed, as shown by polarized optical microscopy. Meanwhile, the regime III–regime II transition temperature was about 15° higher for all of the LCB‐iPP compositions than that of iPP because the LCB structure reduced the mobility of the polypropylene chains. Furthermore, the γ‐form crystal structure was favored by LCB compared to the β form, which was supported by wide‐angle X‐ray diffraction. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Effect of chain disentanglement on melt crystallization behavior of isotactic polypropylene

Isotactic polypropylene (iPP) with “disentangled” chains was generated through crystallization of iPP from its mineral oil solution. TGA test assured complete removal of mineral oil from iPP precipitates. Time sweep rheological measurements showed the modulus build-up with time indicating the formation of “disentangled” chains in iPP after the sample disentanglement treatment. The “disentangled” chains could preserve for a certain time before completely re-entangled during melting. Crystallization kinetics of iPP with “disentangled” chains was studied by using polarized optical microscope. The growth rate of spherulites in “disentangled” iPP was faster than that in the entangled one.     

Fabrication and morphology development of isotactic polypropylene nanofibers from isotactic polypropylene/polylactide blends

The excellent characteristics of polymeric nanofibers with diameters less than 1 μm such as the enormous specific surface result in a dramatic increase in a variety of functional applications. In this article, polymer blends of isotactic polypropylene (iPP) and polylactide (PLA) were fabricated through a twin‐screw extruder. The extrudates were prepared at various processing conditions and the iPP nanofibers were obtained by removal of the PLA matrix from the drawn samples. The influences of drawing ratio, the processing temperature, and the blend ratio of iPP/PLA on the morphology development of iPP phase were investigated by scanning electron microscopy. It was found that the uniformed iPP nanofibers with averaged diameters less than 500 nm were fabricated by the suitable processing parameters. Otherwise, the processing immiscibility and rheological behavior of iPP/PLA blends were studied by means of dynamic mechanical analysis and capillary rheometer. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Effect of solvents on morphology and polymorphism of polyvinylidene fluoride membrane via supercritical CO2 induced phase separation

Poly vinylidene fluoride (PVDF) membranes were prepared via supercritical CO₂ induced phase separation. The effects of solvent power on PVDF membrane morphology and polymorphism were investigated using N‐N‐dimethylformamide (DMF), triethyl phosphate (TEP), and their mixture respectively. The morphology evolution including cross‐section and surfaces were thoroughly studied by scanning electron microscope (SEM) and atomic force microscopy (AFM). The differences of solubility parameters between the solvent and PVDF affected the phase separation and the resultant morphology. The various crystalline phases of the membranes were mainly investigated by Fourier transform infrared spectroscopy (FTIR) and X‐ray diffractometer (XRD). Solvent with larger dipole moment tended to form polar β phase. Decreasing the difference of solubility parameters favored the formation of α phase. Furthermore, the effects of salt additive on PVDF membrane morphology and crystalline form were studied as well. Results turned out that lithium chloride (LiCl) induced a porous top surface and boosted the formation of β phase. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41065.     

Upcycling by grafting onto semi-crystalline polymers using supercritical CO2

The creation of graft copolymers by selectively grafting a second polymer to the amorphous fraction of a semi-crystalline polymer in supercritical CO₂ is demonstrated herein. The graft copolymer is synthesized by free radical polymerization of a vinyl monomer within the semi-crystalline polymer below its melt temperature. Such conditions afford selective grafting on the amorphous regions (block “B”) while leaving the crystalline domains (block “A”) unmodified. Accordingly, unique A-B, A-B-A, A-B-A-B-A, and so forth. block structures are formed. In this work, styrene is polymerized within polyamide 6, polyethylene terephthalate, and isotactic polypropylene. Purification of these material is performed to remove the un-grafted homopolymer, allowing for determination of the graft yield, the portion of polymer which covalently bonds to the semi-crystalline matrix. Grafting yields achieved in polyamide 6, polyethylene terephthalate, and isotactic polypropylene were 98%, 59%, and 15%, respectively. Property enhancements were observed upon further characterization of polystyrene-polyamide 6 copolymers, including high glass transition temperatures, the ability to be remelted, and tunable grafting molecular weight. Additionally, hydrophobicity is controlled by varying polystyrene composition. The remarkable range of accessed properties demonstrates this as a potential route to upcycling plastics.     

New insight into the action of supercritical carbon dioxide for grafting of maleic anhydride onto isotactic polypropylene by reactive extrusion

The reactive extrusion process of isotactic polypropylene (iPP) grafting maleic anhydride (PP-g-MAH) initiated by dicumyl peroxide (DCP) in the presence of supercritical carbon dioxide (scCO₂) is investigated. Because of its moderate supercritical conditions and well swollen performance in iPP melts, scCO₂ is selected to be imported into the extruder system to reduce process temperature and melt viscosity as well as strengthen the mass transport. It has been found that the process temperature of reactive sections of co-rotating twin screw extruder can be successfully reduced from conventional 190 to 160 °C when assisted with the addition of scCO₂. Consequently, effective suppression of main chain degradation is observed, which leads to the products with relative higher molecular weight and narrower molecular weight distribution. The experimental results also indicate a significant increase in both the grafting degree of resultant PP-g-MAH and the grafting efficiency of MAH under certain operation conditions. Especially, the grafting efficiency is close to 90% when low concentration of both MAH and DCP are employed. A potential mechanism has been proposed to explain the effects of scCO₂ in the reactive extrusion. In comparison with traditional molten grafting process, the work presents a novel approach to increase the grafting efficiency of MAH and control the molecular weight of resultant PP-g-MAH simultaneously.     

Polymerization kinetics of styrene in isotactic polypropylene pellets and evolution of phase morphology during polymerization

Polymerization kinetics of styrene (St) in commercially available isotactic polypropylene (iPP) pellets and the phase morphology evolution during polymerization are investigated. The polymerization rate of St in iPP pellets is slightly faster than in the corresponding bulk and suspension polymerizations carried out under similar reaction conditions due to the existence of two reaction sites: amorphous PP and polystyrene (PS), which are formed by polymerization‐induced phase separation. Two mechanisms are proposed for the phase morphology evolution: nucleation and growth, and St‐assisted coarsening of phase structure. During polymerization, the size of the dispersed PS particles increases with polymerization time no matter at which position of the pellet, but the increasing amplitude is much bigger at 200 μm distance to the edge than at the center due to much more significant occurrence of St‐assisted coarsening of phase structure which is attributed to both high values of PS/PP and St/PP resulted from polymerization‐induced diffusion of St. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43934.     

On the lamellar morphology of isotactic polypropylene spherulites

The lamellar structures within melt-grown spherulites of the monoclinic form of isotactic polypropylene have been studied by transmission electron microscopy following permanganic etching. Spherulites grown at 160°C are composed solely of α laths which develop in the classical radiating branching manner from axialitic precursors. The branching units are not, however, fibres but individual dominant lamellae as previously found for polyethylene and isotactic polystyrene. The theory of instability of planar interfaces of Keith and Padden is not applicable to this textural situation, although local diffusion fields may add an additional structural dimension to spherulites which form quickly at, say, 130°C. Instead, one may possibly look to pressure from compressed cilia as the cause of splaying between adjacent lamellae. Cross-hatching is present in spherulites grown at 155°C and below in the early stages, and thereafter is concentrated in specific locations and subsidiary lamellae. The cross-hatching members are less stable, probably because they are composed of shorter and/or less tactic molecules.     

Structure and morphology of isotactic polypropylene functionalized by electron beam irradiation

The structure and morphology of isotactic polypropylene (iPP), functionalized by electron beam irradiation at room temperature in air, are investigated by elementary analysis, FT‐infrared (FTIR) spectroscopy, electron spectroscopy for chemical analysis (ESCA), polariscope, and static contact angle. Elementary analysis reveals that the element oxygen has been introduced onto iPP chains after electron beam irradiation. In addition, as shown from FTIR spectra, oxygen‐containing groups, such as carbonyl, carboxyl, and ether groups, are introduced onto iPP molecular chains. The dependence of oxygenation extent (as measured by O_1S/C_1S value of ESCA spectra) on electron beam dose is obtained. Under polariscope, it can be observed that the dominant alpha phase appears to become more enhanced, and there is no crystalline phase transition. The static contact angle of iPP decreases with increasing dose. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 75–82, 2000     

Supercritical CO2 assisted ternary-monomer grafting copolymerization of polypropylene

Free-radical grafting of ternary-monomer onto polypropylene (PP) particles in the solid state has been studied using supercritical carbon dioxide (SC CO₂) as a solvent and a swelling agent. The PP particles were first swelled with the monomers and AIBN as an initiator, using SC CO₂ at different experimental conditions of pressure, temperature and treatment time. After releasing CO₂, monomers were grafted onto PP in different temperatures. During monomer selection, combination of soft monomer and hard monomer was used in order to tune polarity and flexibility of grafted polymer. FTIR spectra confirmed that ternary-monomer had been grafted onto PP and SEM showed that grafted molecules had been uniformly distributed in the PP substrate. TG analysis indicated that thermal stability of grafting modified PP had been improved, and DSC revealed that grafting leads to a lower degree of crystallinity of polypropylene.     

The spherulitic and lamellar morphology of melt-crystallized isotactic polypropylene

A study is presented concerning the basic morphology of melt-crystallized isotactic polypropylene (iPP). Involved within, is the coordinated application of optical and electronmicroscopy on a range of commercial iPP-s, crystallized in the temperature range 100°C–150°C. For electron microscopy in particular, the permanganic etching technique has been used throughout, providing the simultaneous combination of both real space microstructures with electron diffraction information. The investigation itself has centred on the five different spherulite types, as identified optically, which were then correlated with the details of their particular lamellar morphology. It was found that each spherulite type is characterized by virtue of the arrangement of its constituent lamellae, in terms of orientation, habit type and crystal structure. Thus, specific correlations were obtained between the structural entities on all scales of the structure hierarchy.     

Free‐radical grafting of acrylic acid onto isotactic polypropylene using styrene as a comonomer in supercritical carbon dioxide

Free‐radical grafting of acrylic acid (AAc) onto isotactic polypropylene (iPP) using styrene (St) as a comonomer in supercritical carbon dioxide (SCCO₂) medium was studied. The effects of temperature and pressure of reaction on functionalization degree (grafting degree of AAc) of the products were analyzed. The increase of reaction temperature increases the diffusion of monomers and radicals in the disperse reaction system of SCCO₂. In addition, the increase of temperature accelerates the decomposition rate of 2,2′‐azobisisobutyronitrile (AIBN), thus promoting grafting reaction. It was also observed that functionalization degree of the products decreases with the increase of pressure of SCCO₂ in the range of experiment. The effects of comonomer St on the functionalization degree of the products were investigated. The AAc graft degree of the resulting polymer was drastically higher in the present of St. It reached a maximum when the mass ratio of St and AAc was about 0.7 : 1. Because AAc is not sufficiently reactive toward iPP macroradicals, it would be helpful to use a second monomer that can react with them much faster than AAc. St preferentially reacts with the iPP macroradicals to form more stable styrene macroradicals, which then copolymerize with AAc to form branches. The highest functionalization degree was obtained when the AIBN was 0.75 wt %. When the initiator was used excessively, the functionalization degree decreased because of severe chain degradation of the iPP backbone. The morphologies of pure iPP and grafted iPP are different under the polarizing optical microscope. The diameter of the pure iPP spherulites is 20–38 μ and that of the grafted iPP spherulites is reduced with the increase of the functionalization degree of the products. This is proposed to be because the polar grafts formed during the reaction would have a tendency to associate in the hydrophobic PP environment. This might preserve some of the local crystalline order that existed during the reaction in the swollen iPP phase. It can be proven by a DSC cooling investigation that the crystallization temperature increased as the functionalization degree increased. This is proposed to be because the side‐chain of grafting polymer helps to bring about the heterogeneous nucleation in grafting polymer. Therefore, a large number of nuclei can emerge to a lesser supercooling degree. It can be also proven that the percent crystallization decreased as the functionalization degree increased, probably due to the grafted branches, which disrupted the regularity of the chain structure and increased the spacing between the chains. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 2203–2210, 2004     

Enhanced foamability of isotactic polypropylene/polypropylene-grafted-nanosilica nanocomposites in supercritical carbon dioxide

Polypropylene-grafted nanosilica (PP-g-SiO₂) was prepared by us as a new modified nanosilica with long polymer chains and high grafting density. It was found that the addition of PP-g-SiO₂ resulted in remarkable strain-hardening behavior of PP. Herein, the foaming behavior of isotactic polypropylene (iPP)/PP-g-SiO₂ nanocomposites was investigated by using supercritical carbon dioxide (scCO₂) as a blowing agent. The results demonstrated that the incorporation of PP-g-SiO₂ could obviously enhance the foamability of iPP. In particular, the uniform cell distribution and smaller cell size could be obtained by 1 wt% particle loading, and 5 wt% particle content showed a wider foaming temperature range and higher cell density. The noticeable enhancement in the foamability of iPP was attributed to the reinforced melt strength, high melt elasticity and the increased heterogeneous nucleation caused by well dispersed and long polypropylene chains grafted SiO₂. These findings provide new insights to improve the foaming ability of iPP with incorporation of modified nanoparticles.     

Different factors in the supercritical CO2‐assisted grafting of poly(acrylic acid) to polypropylene

The grafting of poly(acrylic acid) to polypropylene was realized with supercritical CO₂ as a substrate swelling agent and a monomer/initiator carrier. The effects of different supercritical CO₂‐assisted impregnation conditions on the substrate mass increment and grafting efficiency were studied. The original isotactic polypropylene and the grafting product were characterized through IR spectroscopy, differential scanning calorimetry, and scanning electron microscopy. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 4280–4285, 2006     

Synthesis of multifunctional polypropylene via solid phase cografting and its grafting mechanism

Functional monomers, maleic anhydride (MAH) and vinyl acetate (VAc), were used as comonomers in the solid‐phase grafting of polypropylene (PP). Quantitative determination of the graft level of both MAH and VAc performed by titration and FTIR methods allowed for a good appreciation of the interaction of MAH and VAc. A product with high‐graft level of both VAc and MAH was obtained when the feeding molar ratio of MAH/VAc approached 1 : 1. The melt flow rate (MFR) of the grafted PP decreased with the increase in the feeding amount of VAc. It was proposed that MAH and VAc molecules could form a kind of stable transition state during the graft polymerization and that the reactivity of both monomers were enhanced. The resulting multifunctional PP offers possibility for the development of novel PP‐based polymer blends and composites, thus extending the application field of PP. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 929–935, 2002; DOI 10.1002/app.10121