The influence of stereoregularity defects on crystals of isotactic poly(propylene)

The local conformation and the defect Gibbs energy of stereoregularity defects in isotactic poly(propylene), i-PP, is calculated by means of the thermodynamic-integration approach. For i-PP with high isotacticity, where the atactic inclusions do not interfere with each other, we found changes in the torsion angles next to the defect, which bring the methyl unit to the position it would have in the conformation of the purely isotactic chain. The Gibbs energy of this inclusion is low and enables cocrystallization corresponding to the uniform inclusion model. For sequences of adjacent defects in the polymer chain, the local chain deformation was found to be much more complex, and a considerable defect Gibbs energy remains that makes the inclusion of atactic sequences less likely.     

Structure-property relationships in isotactic poly(propylene)/ethylene propylene rubber/montmorillonite nanocomposites

Nanocomposites based on isotactic polypropylene/ethylene propylene rubber (iPP/EPR) were prepared adding different amounts of montmorillonite and maleated polypropylene. The structure and morphology of the samples were characterized by small angle X-ray scattering, wide angle X-ray diffraction, electronic and optical microscopy and differential scanning calorimetry, iPP showed a polymorphic behavior. Clay disrupted the ordered crystallization of iPP and had a key role in shaping the distribution of iPP and EPR phases: larger filler contents brought about smaller, less coalesced and more homogeneous rubber domains. Clay distributed itself only in the continuous phase and not in the rubber domains. Tactoids persisted on the surface of the sample, while delamination proceeded to a greater degree in the bulk of the materials. Melt flow rate, impact strength, flexural and tensile properties, were also measured and a structure-property correlation was sought. Clay produced its most significant effect on physical-mechanical properties by controlling the size of rubber domains in the heterophasic matrix. This allowed to obtain nanocomposites with increased stiffness and impact strength, a remarkable achievement for polymer layered-silica nanocomposites that usually suffer the drawback of being stiffer than the unfilled matrix, but at the same time with a lower resistance to impact. A beneficial effect of clay on thermal stability was also observed.     

In Situ Fourier transform infrared spectroscopic study of the thermal degradation of isotactic poly(propylene)

The conformational change of isotactic poly(propylene) (iPP) during the thermal degradation process has been carefully studied by in situ Fourier transform infrared (FT-IR) spectroscopy. This new method of studying thermal degradation of iPP not only shows the conventional kinetic parameter information of thermal degradation such as the degradation activation energy DeltaE and the degradation factor n, which are in accord with the results of traditional thermogravimetry experiments, but also indicates that many significant molecular structure changes occur during the thermal degradation process that come from some characteristic absorption band changes of in situ FT-IR. A multivariate approach, principal components analysis (PCA), is applied to the analysis of infrared (IR) data, and the results further confirm the multi-step processes of the thermal degradation of iPP. Above all, this is a new application to polymer thermal degradation by in situ FT-IR that connects the intermediate conformational change with final results during thermal degradation.     

Simple model of particle formation by homogeneous and heterogeneous nucleation

Particle formation processes by homogeneous and heterogeneous nucleationare becoming increasingly important for the preparation of functional materials. However, the complexity of the presently available nucleation theory, which considers the classical thermodynamics of the process, prevents its practical use. To overcome this, a simple and practical engineering model of particle formation by homogeneous and heterogeneous nucleation in the liquid phase has been proposed, which can predict the number concentration of nucleated particles as a function of the operating conditions. If the number concentration of nucleated particles can be predicted, their mean diameter can be determined from a mass balance. Experiments in which the precursor monomers were generated by several methods have successfully confirmed the predictions of the model. Furthermore, the original model developed for liquid-phase nucleation is extended to the case in which the precursor monomers are generated in a gas-phase system. A relationship between number concentration and mean volume diameter of nucleated aerosolparticles and operating conditions is obtained considering the free molecular regime vapor condensation onto the critical nuclei, which is the main difference with the liquid-phase model. The predictions are in excellent agreement with the results obtained in gas-phase nucleation experiments. In the present paper, the simple model and the comparison of experimental results with the theoretical predictions are summarized.     

Heterogeneous nucleation and self-nucleation of poly(p-dioxanone)

The changes in nucleation behaviour upon addition of Boron Nitride (BN), Talc and Hydroxyapatite (HA) to poly(p-dioxanone) (PPDX) were monitored by DSC and Polarised Optical Microscopy (PM). Self-nucleation DSC studies evidenced the existence of the usual three self-nucleation domains depending on the self-nucleation temperature (T _s) employed. By far the best nucleation agents for PPDX were its own self-nuclei and this result was independent of the presence or absence of any of the other nucleating agents employed; once Domain II was reached, self-nucleation dominated the nucleation process. BN and Talc were able to nucleate PPDX, thereby increasing its nucleation density, its dynamic crystallisation temperature upon cooling from the melt (T _c) and its enthalpy of crystallisation (H _c). BN was a better nucleating agent than talc. HA on the other hand caused an antinucleation effect on PPDX characterised by a decrease in its nucleation density, a decrease in its T _c and in H _c. Isothermally crystallised PPDX exhibited large banded spherulites whose morphology changed as a function of crystallisation temperature from single banded structures with a very clear Maltese cross to double banded spherulites. PPDX also shows a change in growth regime upon increasing crystallisation temperature (from Regime III to Regime II) according to the kinetic interpretation of growth rate data. BN did not cause any significant modification of the spherulitic growth kinetics (in Regime II) except for a small decrease in surface free energy of PPDX crystals (_e). On the other hand HA was found to increase the spherulitic growth rate and the overall crystallisation rate of PPDX, this increase was caused by a degradation process experienced by the polymer during the treatments involved in isothermal crystallisation that was only present in the samples with HA. It is postulated that the interaction between the phosphate groups on the surface of HA and the ester groups of PPDX are responsible for both the antinucleation effect and the catalysis of the hydrolytic degradation of PPDX.     

Preparation of polymeric micelles consisting of poly(propylene glycol) and poly(caprolactone)

The ring-opening polymerization of epsilon-caprolactone (CL) was carried out with polypropylene glycol (PPG) as an initiator in the presence of the monomer activator HCl. Et2O to synthesize poly(epsilon-caprolactone)-poly(propyleneglycol)-poly(epsilon-caprolactone) (PCL-PPG-PCL) triblock copolymers with change of length PPG and PCL. The micelle formation of PCL-PPG-PCL triblock copolymers in an aqueous phase was confirmed by NMR, dynamic light scattering and fluorescence techniques. The critical micelle concentration (CMC) of the PCL-PPG-PCL triblock copolymers, determined from fluorescence measurements, was in range of 1.4 x 10(-3)-4.6 x 10(-3) mg/ml with dependence on block lengths of PPG and PCL. The partition equilibrium constant, K(v), which is an indicator of the hydrophobicity of the micelles of the PCL-PPG-PCL triblock copolymers in aqueous media, was also changed with dependence on length PPG and PCL. We confirmed that the PCL-PPG-PCL triblock copolymers formed micelles and hence may be potential hydrophobic drug carriers.     

Aerogels and polymorphism of isotactic poly(4-methyl-pentene-1)

Monolithic and highly crystalline aerogels of isotactic poly(4-methyl-pentene-1) (i-P4MP1) have been prepared by sudden solvent extraction with supercritical carbon dioxide from thermoreversible gels. The cross-link junctions of i-P4MP1 gels, depending on the solvent, can be constituted by pure polymer crystalline phases (I or III or IV) or by polymer-solvent cocrystalline phases (for cyclohexane and carbon tetrachloride gels). Gels with cocrystalline phases lead to aerogels exhibiting the denser crystalline form II, whereas all the other considered gels lead to aerogels exhibiting the thermodynamically stable form I. Aerogels obtained from form I gels, which do not undergo a crystalline phase transition during the CO(2) extraction process present the high structural stability most suitable for the preparation of porous membranes. The effect of solvents on the aerogel pore structure and morphology has been also investigated by scanning electron microscopy and N(2) sorption measurements. In all cases, the aerogels present highly porous interconnected structures with macropores and mesopores presenting a large size distribution and a vanishing presence of micropores.     

Optical emission from confined poly(thiophene) chains

We discuss the stages of the luminescence processes in substituted and soluble poly(thiophenes), using results from determinations of absolute quantum yields of photoluminescence, fast pump-probe experiments and microcavity devices. Enhancement of the quantum yield of photoluminescence, with poly(thiophenes) in the solid state, can be obtained by dispersing the conjugated chain in a molecularly dispersed polymer blend, or by adding side chains designed for forcing the conjugated main chains apart. At the wavelength of stimulated emission, we observe a narrow bandwidth emission in the high-Q one-dimensional microcavity devices prepared by sandwiching two dielectric mirrors around a thin polymer film. A strong enhancement of the power efficiency is observed at a pump power of 1 μW/cm², but does not conclusively show lasing characteristics.     

聚碳酸亚丙酯/聚甲基丙烯酸甲酯复合材料热降解动力学研究

利用溶液共混法制备聚碳酸亚丙酯/聚甲基丙烯酸甲酯(PPC/PMMA)复合材料,采用热重分析法研究PPC/PMMA热性能,结果表明PMMA的加入可以有效提高PPC的热稳定性能。以多升温速率法对PPC/PMMA热降解动力学进行研究,利用Friedman法、Kissinger法、Flynn-Wall-Ozawa法、Coats—Redfem方法对热降解动力学数据进行分析,确定PPC/PMMA热分解反应机理函数为Avrami-Erofeev方程,遵循一种热降解机理。     

Voltage-dependent fabrication of nanoporous alumina and the applications to nanocapacitors

We investigated the fabrication of anodized aluminum oxide by anodization processes under DC and AC voltage biases. A two-step anodization process was used to fabricate the anodized aluminum oxide dielectrics in order to regulate the ordered nanopore array at the surface. AC samples showed the distorted nanoporous structure instead of a straight nanopore array in DC samples. As increasing the frequency of AC bias the nanovoid or nanocavity structure was formulated with the increased density of nanovoids. Nanoporous alumina was used for the fabrication of Ni-insulator-Al capacitors. The DC sample shows the tunneling process of an increase in leakage current and breakdown. When a negative voltage is applied to the capacitor device, the small current of 2 nA flows at a voltage of 0 V, indicating the existance of residual leakage current. The AC sample had very low leakage currents of the AAO dielectrics and the AAO hexagonal unit cell formed the nanocapacitor with a capacitance of 1-2 aF.     

Elastic properties of highly anisotropic thin poly(propylene) foams

In this letter, elastic properties of highly anisotropic cellular poly(propylene) films are reported. The material shows peculiar elastic properties compared to other foams in the literature. The data is displayed as the relative Young's modulus E*/E_s versus relative density ρ*/ρ_s. Almost all the data from the literature are located on the region E*/E_s=(ρ*/ρ_s)ⁿ with 1≤n≤6. The introduced material on the other hand has lower relative Young's modulus at high relative densities, n≥6.     

Morphology development of isotactic poly(4-methylpentene-1) during melt crystallization

The morphology of melt-crystallized isotactic poly(4-methylpentene-1) samples, prepared by varying crystallization temperature and time, as well as annealing temperature, has been probed by small and wide angle x-ray scattering, and electron microscopy. This was then correlated with the thermal properties investigated by differential scanning calorimetry. Relationships among melting behavior, morphological evidences and X-ray results have been found. Different populations of lamellae, grown with a time-dependent sequence, can be obtained: their relative amount and thermal stability is determined by crystallization and annealing conditions.     

Chemisorbed poly(propylene sulphide)-based copolymers resist biomolecular interactions

Alkanethiolates have been widely used as chemisorbates to modify gold surfaces, in spite of their relatively poor oxidative stability. We introduce gold-chemisorbing block copolymers bearing an anchoring block of poly(propylene sulphide) (PPS), selected in the expectation of greater stability. These materials offer a more robust approach to surface modification of gold. As an example, a triblock copolymer with poly(ethylene glycol) (PEG) was selected, with the goal of minimizing biological adsorption and adhesion. The copolymer PEG17-bl-PPS25-bl-PEG9 chemisorbed to form a dense monolayer of 226 +/- 26 ng cm(-2), approximately 2.2 nm thick. The copolymeric adlayer was much more stable to oxidation than commonly used alkanethiolates. Its presence greatly reduced protein adsorption (>95%), even after exposure to whole blood serum (>55 mg x ml(-1)), as well as cell adhesion over long culture durations (>97%). PPS-containing copolymers are an attractive alternative to alkanethiolates, and PEG-bl-PPS-bl-PEG presents a powerful example for use in biodiagnostic and bioanalytical devices.     

可降解聚碳酸亚丙酯马来酸酯载药微球的制备

采用阴离子配位聚合方法,合成了二氧化碳(CO2),环氧丙烷(PO)与马来酸酐(MA)的三元共聚物,聚碳酸亚丙酯马来酸酯 (PPCMA).采用复相乳液(W/O/W)溶剂挥发法制备了包裹水溶性模型药物葡萄糖(glucose)的可降解微球,并研究了壁材与囊心的比例、稳定剂明胶浓度、搅拌速率等因素对微球性能的影响.当v(PPCMA)∶v(glucose)=1∶2,gelatin质量分数为0.2%,第1次乳化搅拌速率为400r/min,第2次乳化搅拌速率为500r/min时,得到粒径较小、载药量和包封率分别为26.1%和76.1%的载药微球.