Microcellular silicone rubber foams: The influence of reinforcing agent on cellular morphology and nucleation

A series of microcellular silicone rubber foams were prepared by supercritical carbon dioxide (scCO₂). The effect of reinforcing agent (silica) on the rheological behavior, cellular morphology and nucleation of silicone rubber composites was investigated. The results show that the silica not only acts as reinforcing agent but also plays an important role in the cellular nucleation. When the content of silica increases from 40 phr to 70 phr, the range of the calculated surface tension of silicon rubber composites/scCO₂ (γ_mix) and radius of the critical nucleus (r*) are 158.95～1,092.74 nN/m and 14.45–99.34 nm, respectively. Meanwhile, aggregated silica has good heterogeneous nucleation as the diameter of aggregated silica particles is approximate to twice r*. The smallest cell diameter and the highest cell density of the silicone rubber foam can reach to 708 nm and 1.02 × 10¹¹ cells/cm³, which indicate that the silicone rubber nanofoams can be obtained by means of the supercritical foaming technology. POLYM. ENG. SCI., 59:5–14, 2019. © 2018 Society of Plastics Engineers     

Fracture behavior of polyetherimide (PEI) and interlaminar fracture of CF/PEI laminates at elevated temperatures

To investigate the effects of environmental temperature on fracture behavior of a polyetherimide (PEI) thermoplastic polymer and its carbon fiber (CF/PEI) composite, experimental and numerical studies were performed on compact tension (CT) and double cantilever beam (DCB) specimens under mode‐I loading. The numerical analyses were based on 2‐D large deformation finite element analyses (FEA). Elevated temperatures greatly released the crack tip triaxiality (constraint) and promoted matrix deformation due to low yield strength and enhanced ductility of the PEI matrix, which resulted in the greater plane‐strain fracture toughness of the bulk PEI polymer and the interlaminar fracture toughness of its composite during delamination propagation with increasing temperature. Furthermore, the high triaxiality was developed around the delamination front tip in the DCB specimen, which accounted for the poor translation of matrix toughness to the interlaminar fracture toughness by suppressing the matrix deformation and reducing the plastic energy dissipated in the plastic zone. Especially, at delamination initiation, the weakened fiber/matrix adhesion at elevated temperatures led to premature failure of fiber/matrix interface, suppressing matrix deformation and preventing the full utilization of matrix toughness. Consequently, low interlaminar fracture toughness was obtained at elevated temperatures. POLYM. COMPOS., 26:20–28, 2005. © 2004 Society of Plastics Engineers.     

Effects of processing conditions on foaming behaviors of polyetherimide (PEI) and PEI/polypropylene blends in microcellular injection molding process

Foaming behaviors of both neat polyetherimide (PEI) and PEI/polypropylene (PP) blends were studied in this article in microcellular injection molding (Mucell) process. The study mainly focused on the comparison of two materials' foaming behaviors under different processing conditions which took a critical effect on the morphologies of foams. The results indicated that the different characteristics of PEI and PEI/PP blends, such as melt strength, gas dissolvability, and solubility, induced different nucleation ability of PEI and PEI/PP blends. The addition of PP could obviously improve the cell density and reduce the cell size. With the processing conditions changing, the morphologies of PEI/PP altered more variously, and their distribution of cell density was wider. This suggested that foaming behaviors of PEI/PP blends was more flexible to be controlled by the processing conditions than neat PEI. The effects of shot size, gas injection, and injection rate on foam morphologies were studied in detail. Shot size determined the weight reduction of samples and affected the cell density and size significantly. Gas dosing time and dosing rate determined the gas ratio which effected on foam morphologies of the PEI and PEI/PP foams. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41443.     

Ge quantum dot arrays grown by ultrahigh vacuum molecular-beam epitaxy on the Si(001) surface: nucleation,morphology, and CMOS compatibility

Issues of morphology, nucleation, and growth of Ge cluster arrays deposited by ultrahigh vacuum molecular beam epitaxy on the Si(001) surface are considered. Difference in nucleation of quantum dots during Ge deposition at low (≲600°C) and high (≳600°C) temperatures is studied by high resolution scanning tunneling microscopy. The atomic models of growth of both species of Ge huts--pyramids and wedges-- are proposed. The growth cycle of Ge QD arrays at low temperatures is explored. A problem of lowering of the array formation temperature is discussed with the focus on CMOS compatibility of the entire process; a special attention is paid upon approaches to reduction of treatment temperature during the Si(001) surface pre-growth cleaning, which is at once a key and the highest-temperature phase of the Ge/Si(001) quantum dot dense array formation process. The temperature of the Si clean surface preparation, the final high-temperature step of which is, as a rule, carried out directly in the MBE chamber just before the structure deposition, determines the compatibility of formation process of Ge-QD-array based devices with the CMOS manufacturing cycle. Silicon surface hydrogenation at the final stage of its wet chemical etching during the preliminary cleaning is proposed as a possible way of efficient reduction of the Si wafer pre-growth annealing temperature.     

Necking mechanism and its elimination in uniaxially drawn films of poly(ethylene naphthalate) (PEN)/polyetherimide (PEI) blends

The influence of blend composition on the deformation behavior of cast amorphous PEN/PEI blends were investigated above their respected glass transition temperatures. PEN inherently shows a sharp necking phenomenon when stretched at temperatures as high as 20°C above its glass transition temperature. This was attributed to highly localized rapid alignment of naphthalene planes parallel to the surface of the films. The addition of PEI was observed to reduce this necking behavior. The neck formation completely disappears when the PEI fraction exceeds 10% in the blend. X-ray studies indicate that the increase of PEI hinders the rapid alignment of naphthalene planes parallel to the surface of the films. The presence of PEI chains in the blend was found to increase the overall friction between the polymer chains in the system and this was found to prevent the formation of highly localized necks. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65: 2059–2074, 1997     

Microcellular and nanocellular solid-state polyetherimide (PEI) foams using sub-critical carbon dioxide I. Processing and structure

In this study we investigate the solid-state batch foaming of polyetherimide (PEI) using sub-critical CO₂ as a blowing agent. We report on the gas diffusion for various saturation pressures in this system. Foaming process characterization is reported detailing conditions used to create microcellular and nanocellular PEI foams of 40% and higher relative density. Gas sorption, foaming, and resultant morphologies are analyzed and compared to previously reported results on PEI thin films. It was found that equilibrium gas concentrations for PEI sheet begin to significantly exceed that of films for CO₂ pressures above 3 MPa. A large solid-state foaming process window has been identified that allows for the creation of either microcellular or nanocellular structures at comparable density reductions. A transition from micro-scale cells to nano-scale cells was observed at gas concentrations in the range of 94–110 mg CO₂/g PEI. Additionally, a hierarchical structure was observed which consisted of nanocellular structures internal to microcells. The PEI–CO₂ system offers the unique opportunity to compare and contrast the bulk properties of nanofoams and microfoams.     

Microcellular and nanocellular solid-state polyetherimide (PEI) foams using sub-critical carbon dioxide II. Tensile and impact properties

Microcellular foams, closed-cell polymer foams with cells of order 10 μm, have been studied for over two decades. These foams have shown significant improvements in mechanical properties, such as strength-to-weight ratio, over conventional foams with cells on the order of 1 mm. Will an additional 100-fold reduction in cell size yield further improvement in properties? Here we answer this question in the solid-state PEI-CO₂ system, a unique gas-polymer system in which cellular structures can be created throughout the polymer volume at either micro or nano scales. The tensile and impact behaviors of microcellular (cells in 2-5 μm range) and nanocellular (cells in the 50-100 nm range) structures are experimentally compared for foams with relative densities, to that of the starting solid, in the range of 0.75-0.90. We found that nanofoams show a significantly higher strain to failure, resulting in an improvement in the modulus of toughness by up to 350% compared to microcellular foams. Falling weight impact tests show evidence of a brittle-to-ductile transition in nanofoams resulting in impact energies that are up to 600% higher compared to microcellular foams. These results point to the promise of nanofoams as an important new class of materials.     

Solid-state grafting of maleic anhydride onto polypropylene: The influence of morphology of polypropylene on heterogeneous reaction

Polypropylene (PP) was modified with maleic anhydride (MAH) via heterogeneous solid-state radical grafting performed in a fluidized polymerization reactor. The effect of PP morphology and different concentration of initiator (dibenzoyl peroxide, DBP) on the course of the reaction and grafting efficiency was evaluated with using infrared spectroscopy, scanning electron microscopy (SEM) and particle surface analysis (BET method). With respect to obtained results, the reaction course can be strongly affected by the reaction conditions and PP morphology. Unlike the reactive modification in melt, heterogeneous reaction exhibits lower extent of β-scission albeit the reaction conversion is lower. Solid-state grafting can be used as an alternative method for the surface modification of nonpolar surfaces with polar monomers under mild conditions.     

高温泡沫剂与防膨剂的配伍性研究

研究了高温泡沫剂GFPJ-1与防膨剂CH-1、CH-2的配伍性。一方面,考查了高温泡沫剂GFPJ-1对防膨剂CH-1、CH-2防膨效果的影响;另一方面,研究了防膨剂CH-1、CH-2对高温泡沫剂GFPJ-1发泡性能的影响。结果表明,高温泡沫剂基本上不影响防膨剂的防膨效果,防膨剂对高温泡沫剂的发泡性能有一定的影响;当w（防膨剂）≤0.5%时,防膨剂对高温泡沫剂的发泡性能影响较小,当w（防膨剂）〉0.5%时,防膨剂对高温泡沫剂的发泡性能影响较大。不同防膨剂CH-1、CH-2对于同一种高温泡沫剂的发泡性能具有不同的影响。     

The influence of additives on the primary particle nucleation and agglomeration in poly(vinyl-chloride)

The formation and agglomeration of PVC primary particles were studied in bulk polymerization experiments. In the absence of additives, the primary particles started to agglomerate at low conversions. The agglomeration conversion, as well as the size of the agglomerated particles, decreased when the agitation speed increased. At the highest speed tested, the agglomeration started already at 0.05 percent conversion. The primary particle size was about 0.16-0.18 μm, and seemed to be constant in the conversion interval studied (up to 5 percent). This indicated that the nucleation rate of primary particles was almost constant and that the growth rate of agglomerated particles was very low. The addition of sorbitan monolaurate produced a decrease in primary particle size. Polymeric additives such as PMMA, EVA, and PVAc stabilized primary particles against agglomeration but had no marked effect on the primary particle size. The monomer-soluble fraction of poly(vnyl alcohol-b-vinyl acetate) with high content acetate groups did not affect either the particle size or the agglomeration process.     

Role of epitaxy of nucleating agent (NA) in nucleation mechanism of polymers

The role of “epitaxy” of nucleating agent (NA) in nucleation mechanism of polymers was studied to formulate the nucleation rate (I) as a function of concentration of NA (C_NA) in mixture of polymer and NA and lateral size of a NA crystal (a_NA), I ∝ C_NA/a_NA. It is proved that the epitaxy of NA controls nucleation mechanism by confirming above formula experimentally by observing nucleation by means of optical microscopy. We also clarified that heterogeneous nucleation is a probabilistic phenomenon by confirming that “induction time of nucleation (τ_i)” did not depend on C_NA. We established a method to obtain reliable I and τ_i by adding NA which has been a difficult problem in polymer science.     

水泥颗粒形貌对其性能影响的研究(下)

（接上期）这表明水泥颗粒的球形化，减小了浆体中水泥颗粒与骨料颗粒相对滑动间的摩擦阻力。球形化水泥净浆标准稠度低，表明球形水泥较一般水泥需水量小，颗粒表面所需包裹水层小。我们对比分析了１号至４号样品的水泥颗粒圆形系数提高百分数与相应胶砂流动度增大幅度的关系（见图１１）。由图１１我们可以清晰地看出，水泥颗粒圆形系数提高幅度越大，即颗粒形貌越好，相应的胶砂流动性就越好。从中也可以看出，随水泥颗粒圆形系数的提高，水泥胶砂流动度有不断增大的趋势。水泥需水量低且胶砂和易性好，对混凝土施工很重要。因为在确保混…     

Batch foaming of carboxylated multiwalled carbon nanotube/poly(ether imide) nanocomposites: The influence of the carbon nanotube aspect ratio on the cellular morphology

A series of microcellular poly(ether imide) (PEI) foams and nanocellular carboxylated multiwalled carbon nanotube (MWCNT‐COOH)/PEI foams were prepared by the batch foaming method. MWCNT‐COOHs with different aspect ratios were introduced into the PEI matrix as heterogeneous nucleation agents to improve the cell morphology of the microcellular PEI foams. The effect of the aspect ratio of the MWCNT‐COOHs on the cellular morphology, and gas diffusion is discussed. The results show that with the addition of MWCNT‐COOH, the sorption curve showed a slight reduction of carbon dioxide solubility, but the gas diffusion rate could be improved. The proper aspect ratio of MWCNT‐COOH could improve the cellular morphology under the same foaming conditions, in which m‐MWCNT‐COOH (aspect ratio ≈ 1333) was the best heterogeneous nucleation agent. When the foaming temperature was 170°C, the cell size and cell density of nanocellular m‐MWCNT‐COOH reduced to 180 nm and increased to 1.58 × 10¹³ cells/cm³, respectively. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42325.     

水泥颗粒形貌对其性能影响的研究(上)

水泥颗粒形貌球形化，可以明显改善水泥物理性能。研究表明，水泥颗粒圆形系数（平均）由0．65提高至0．73时，水泥胶砂流动度最大可提高25％，并且随圆形系数的提高，有不断增大的趋势。在相同胶砂流动度下，圆形系数由0．67提高至0．72，W／C可减少8％，且各龄期水泥强度提高均20％以上。球形化水泥的凝结时间比一般水泥长。球形化水泥颗粒堆积密实，水泥胶砂试体中总孔隙率低、中位孔径小，细孔（15．1nm-39nm）或微细孔（3．4nm-15.1nm）含量增多；相同水灰比下中后期强度较一般水泥可提高6－10MPa。     

The influence of talc as a nucleation agent on the nonisothermal crystallization and morphology of isotactic polypropylene: The application of the Lauritzen–Hoffmann,Avrami, and Ozawa theories

A new method is proposed, which can be used to analyze the influence of different additives and fillers on the nonisothermal crystallization of polymers. The composites of talc in isotactic polypropylene (i‐PP) were prepared using a corotating twin‐screw extruder. The compounds were subsequently dried and injection molded. PP morphology and talc dispersion were visualized using optical microscopy and computed tomography. Wide‐angle X‐ray scattering and small‐angle X‐ray scattering measurements provided an insight into the crystal structure of PP. The data obtained from nonisothermal DSC measurements were fitted to the Avrami model for the nonisothermal case. The calculated Avrami's exponent (n), which takes into account the influence of talc on the nucleation and growth of the PP crystals, was used in the combination of Lauritzen–Hoffman and Ozawa models to calculate the nucleation parameter (K_g). A good agreement was found between the model predictions and literature values. The examination shows that the developed model extension gives an expected trend in the case of i‐PP filled with talcs from the same origin but with different particle sizes. Furthermore, it is shown that delaminated talc with a higher specific surface is more efficient in nucleation of i‐PP. Thus, the introduced model extension could be a useful tool for comparing of nucleation ability of different additives in the crystallization of polymers. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Dramatic influence of compatibility on crystallization behavior and morphology of polypropylene in NBR/PP thermoplastic vulcanizates

In this study, the isothermal/nonisothermal crystallization behavior of polypropylene (PP) in acrylonitrile butadiene rubber (NBR)/PP thermoplastic vulcanizates (TPVs) prepared with three different processing methods, the compatibility effect therein, and the mechanism involved were studied. We concluded that the vulcanized NBR particles in TPVs act as heterogeneous nucleation centers and increase the number of nuclei. The crystallization rate of PP thereby increases and the growth of PP spherulites is restrained because of the isolation of vulcanized NBR particles. Since the addition of compatibilizer improves the compatibility of NBR and PP, the smaller and uniformly dispersed NBR particles are obtained, resulting in more and smaller PP crystals as well as higher crystallization rate, compared with Ultra-fine fully vulcanized NBR particles (UFNBR)/PP TPV and NBR/PP TPV without compatibilization. The isothermal crystallization kinetics of PP in TPVs obeys the Avrami equation, whereas the nonisothermal crystallization kinetics is well described by the equation of Mo et al.     

Latex interpenetrating polymer networks based on acrylic polymers. II. The influence of the degree of network compatibility on morphology

Two latex interpenetrating polymer networks, one prepared from a pair of supposedly compatible polymers and the other from an incompatible pair, were investigated using transmission electron microscopy and dynamic mechanical analysis. From the results, it was proposed that both interpenetrating polymer networks consisted of latex particles with essentially coreshell morphologies. Evidence for a core-shell structure was more marked for the materials synthesised from the incompatible polymers. The other polymer pair showed indications of a significant amount of mixing.     

Effect of nucleation mode on the morphology and texture of electrodeposited zinc

The effect of temperature, pH and current density on the morphology and texture of electrodeposited zinc on mechanically polished steel was studied. The electrodeposited zinc had mostly basal (0 0 0 2) and low angle planes (1014, 1013, 1012) parallel to the surface. At pH 2, increasing overvoltage (i.e., increasing current density or decreasing temperature) reduced the percentage of basal plane and increased the percentage of low angle planes parallel to the substrate surface. Increasing overvoltage decreased the zinc crystal size. At pH 4, increasing current density increased the percentage of both basal and low angle planes parallel to the surface, but increased the zinc crystal size. This variation of behaviour at pH 4 was explained by a change in nucleation mode due to hydroxide adsorption. The nucleation mode was determined by comparing dimensionless (i/i _m)² vs (t/t _m) potentiostatic current–time transient graphs with models for instantaneous and progressive nucleation. It was shown that at pH 2, instantaneous nucleation was predominant, whereas at pH 4, it was close to progressive.     

The influence of poly(vinyl alcohol) suspending agents on suspension poly(vinyl chloride) morphology

The requirements for PVC suspension resin have changed considerably in the last few years, so much so that few companies have products on their ranges that are more than 4 or 5 years old. The suspending agent has a crucial influence on the morphology of the resin, so the changes in resin characteristics have largely been achieved by changes in the suspending agent systems. After a brief review of the mechanism of PVC suspension polymerisation, the properties of polymers made using PVOH suspending agents are related to changes in the latter. The effect of variations in PVAc degree of hydrolysis and viscosity are related to changes in surface tension. Methods of achieving higher porosity by using low hydrolysis co-suspending agents are described. It is shown that higher bulk densities can be achieved by delayed addition of the PVOH. Levels of conjugated unsaturation and copolymer distributions are also shown to have important influences.