Transcrystallization of isotactic polypropylene

Transcrystallization of isotactic polypropylene — on polymer, metal, and metal-oxide substrates — has been investigated by hot-stage polarizing microscopy. A measure of the extent of transcrystallization was obtained from a count of spherulites per unit length of substrate, the linear nucleation density. Classical free-energy models were used to interpret variations in linear nucleation densities, in which the degree of supercooling and surface-energy expressions were emphasized.     

Heterogeneous nucleation of isotactic polypropylene

The present work was undertaken to establish rates of primary nucleation for isotactic polypropylene and to put forth a modified classical model for the secondary nucleation process in isotactic polypropylene systems.     

Longitudinal volume viscosity of isotactic polypropylene

The volume flow of isotactic polypropylene (i-PP) with number-average molecular weight M?_n = 103 200, melting temperature T_m = 426 K and glass transition temperature T_g = 238 K, has been measured. The elastic modulus of the longitudinal wave, longitudinal volume viscosity, initial longitudinal volume viscosity, and retardation times, are described for compression rates between ca. 1,0·10^?5 and 200,0·10^?5 s^?1, and at temperatures between 443 K and 473 K as well as pressures up to 150 MPa at which no sign of crystallization was detected. The longitudinal volume viscosity decreases with increasing compression rate and with decreasing volume deformation, the behaviour being in all cases a typical non-equilibrium one. The longitudinal volume viscosity increases with increasing temperature, the volume flow activation energy being about 78,08 kJ/mol.     

Molecular weight determination of isotactic polypropylene by high-sensitivity ebulliometry

The use of a new differential ebulliometer and the determination of the number average molecular weight of unfractionated isotactic polypropylenes is described; the results were reproducible and of satisfactory accuracy. Application of a thermopile with 150 junctions and of suitable devices for the regularity of boiling allowed measurements of number average molecular weights M?_n up to 100000 in good agreement with results obtained by other methods. In this way it is possible to measure M?_n directly, e.g. for reliable determinations of M?_w/M?_n also for polymer samples with low molecular weights and/or wide molecular weight distributions.     

Thermal properties of melt and solution crystallized isotactic polypropylene

A differential scanning calorimeter has been used to measure the heat capacity of both melt and solution crystallized samples of isotactic polypropylene from 200 to 500°K. An estimate has been made of heat capacity values below 200°K. Values of the heat capacity, entropy, enthalpy, and free energy have been estimated for 100% crystalline isotactic polypropylene and are listed at 10°K intervals. The glass transition occurs at about 265°K in both samples. A value of 207 ± 5 J?g^?1 is obtained for the heat of fusion of isotactic polypropylene.     

Characterization of blends of high density polyethylene with isotactic polypropylene

Isotactic polypropylene and high density polyethylene are thermodynamically incompatible with one another yet it is possible to prepare physical blends of the two polymers and fabricate solid test specimens with a high degree of cohesion. This paper describes the preparation of such specimens and the investigation of their structure. Direct and indirect evidence builds up a model for the blends involving the degree of dispersion. The techniques include optical microscopy, density measurements and differential scanning calorimetry (DSC). Each blend, formed by deposition from solution, is a fine dispersion of the minor component in the major component. The degree of crystallinity of a blend is identical to that calculated from the crystallinities of the pure components in the appropriate ratio. During melting of the blends there is little interaction between the components but there are mutual effects during recrystallization. The α and β forms of polypropylene arise under appropriate conditions.     

Thermal properties of isotactic polypropylene quenched from the melt and annealed

DSC investigations on isotactic polypropylene (it-PP) quenched from the melt and annealed over a range of temperatures are reported. The results confirm the existence of saturation levels of the ordered phase for different annealing temperatures, previously revealed by X-ray diffraction methods. Thermal, X-ray diffraction, and density data are correlated in terms of the extent of the ordered phase present in the it-PP.     

Electrochemistry of thin polypyrrole films

Thin polypyrrole (PP) films with octanesulfonate, dodecylsulfate and polystyrenesulfonate counterions were electrochemically switched between the conducting oxidized and the insulating neutral state in various electrolytes. In propionitrile under rigorous exclusion of oxygen and water PP can be cycled more than a thousand times without an appreciable decrease in electrochemical capacity. In an aqueous medium this process becomes irreversible after less than a hundred scans, the effect being strongly influenced by the pH of the electrolyte. Furthermore a pronounced dependence of the available charge upon the time (t₅) between subsequent scans is found. It is known that treatment of PP films by a base leads to a remarkable decrease in conductivity. A mechanism which explains the loss of conductivity is proposed, in addition the possible occurrences of irreversible chemical changes as side reactions during electrochemical cycling is discussed.     

Crystallization and thermal behaviour of blends of isotactic polypropylene and hydrogenated oligo(cyclopentadiene)

The crystallization and the thermal behaviour of thin films of isotactic polypropylene/hydrogenated oligo(cyclopentadiene) of low molecular weight (iPP/HOCP) are studied using optical microscopy and differential scanning calorimetry (DSC). The spherulite growth rate, the overall crystallization rate and the equilibrium melting temperature of iPP are decreased by the addition of HOCP to iPP. This leads to the hypothesis that iPP and HOCP form a miscible blend in the amorphous phase. This hypothesis is also supported by the detection of a single blend-composition-dependent glass transition temperature of each blend as determined by DSC.     

Properties and biocompatibility of polypropylene graft copolymer films

Modifying the surfaces of polymers has received a great deal of attention, because it could bring about specific surface characteristics such as antithrombogenic property. Therefore, N-vinyl pyrrolidone/sodium acrylate (NVP/Na-AAc) binary monomers were introduced onto polypropylene (PP) films by a radiation grafting method. The effect of solvent and comonomer composition on the degree of grafting was determined. Studies of the mechanical properties and water content of such graft copolymers showed that as the grafting yield increases the elongation percent decreases. However, the water content increases with increasing grafting yield. The blood compatibility of the original PP and PP-g-NVP/Na-AAc films was evaluated by determination of the extent of platelet adsorption and thrombus formation. The blood compatibility of PP-g-NVP/Na-AAc seems to be better than that of original PP.     

Microstructure and melting point of stereoregular polymers. A tentative experimental correlation for isotactic polypropylene

Fractions of predominantly isotactic polypropylene with varying stereoregularity were examined by NMR analysis and by hot-stage optical microscopy, and the stereochemical composition and the melting point of each fraction were determined. From the stereochemical composition the average length of the isotactic stereoblocks is evaluated, and the experimental results are compared to the lengths calculated according to two statistical models of stereospecific propagation. The experimental melting points are then related to the experimental average lengths of isotactic stereoblocks, and the correlation is discussed.     

Morphology,crystallization, and thermal behaviour of isotactic polypropylene/low density polyethylene blends

The morphology, the crystallization and thermal behaviour of isotactic polypropylene (IPP) in its blends with two different samples of low density polyethylene (LDPE) was investigated at temperatures high enough to prevent any solidification of LDPE. It is found that pre-existing liquid LDPE domains are incorporated in intra-spherulitic regions during the isothermal crystallization of iPP. The radial growth rate of spherulites is almost unaffected by the LDPE content. The overall rate of crystallization of iPP, on the contrary, is strongly depressed by the addtion of LDPE. A depression of the equilibrium melting temperature of iPP, due to kinetic and morphological effects, is also observed. The depression of the overal kinetic rate constant is accounted for by the negative effect (decrease in the number of nuclei) that the addition of LDPE has on the primary nucleation process of iPP.     

Deformation of extremely thin polyethylene films

In order to establish a better correlation between the drawing mechanism of bulk polyethylene (PE) and the observation of plastic deformation of single crystals and the ensuing microfibril formation extremely thin PF films floating on hot water were deformed and studied by electron microscopy. The drawn film consists of about 300 Å wide folded chain blocks arranged in microfibrils. The chains are oriented in the microfibril axis. The lateral coalescence of the blocks into lamellae produces a surface structure of the same type as observed with bulk samples. One hence concludes that also in the latter case the destruction of the original spherulitic structure in the necking zone first produces microfibrils composed of blocks broken off a single lamella. The bundle of microfibrils from a single stack of lamellae with similar draw ratio and draw rate laterally coalesces forming a fibril, i.e., a stack of mosaic type lamellae with rather irregular fold containing surfaces and extending over more or less the whole cross section of the fibril. In contrast with bulk samples, but in agreement with the single crystal deformation, the long period of the drawn film agrees with that of the starting material. The same situation exists with annealing which increases the long period and the order in lamellar orientation and long period, but does not produce any crystal rotation about an axis in the sample surface and perpendicular to the draw direction which is so conspicuous with drawn bulk PE.     

Biocompatibility of atomic layer-deposited alumina thin films

Presented in this paper is a study of the biocompatibility of an atomic layer-deposited (ALD) alumina (Al2O3) thin film and an ALD hydrophobic coating on standard glass cover slips. The pure ALD alumina coating exhibited a water contact angle of 55 degrees +/- 5 degrees attributed, in part, to a high concentration of -OH groups on the surface. In contrast, the hydrophobic coating (tridecafluoro-1,1,2,2-tetrahydro-octyl-methyl-bis(dimethylamino)silane) had a water contact angle of 108 degrees +/- 2 degrees. Observations using differential interference contrast microscopy on human coronary artery smooth muscle cells showed normal cell proliferation on both the ALD alumina and hydrophobic coatings when compared to cells grown on control substrates. These observations suggested good biocompatibility over a period of 7 days in vitro. Using a colorimetric assay technique to assess cell viability, the cellular response between the three substrates can be differentiated to show that the ALD alumina coating is more biocompatible and that the hydrophobic coating is less biocompatible when compared to the control. These results suggest that patterning a substrate with hydrophilic and hydrophobic groups can control cell growth. This patterning can further enhance the known advantages of ALD alumina, such as conformality and excellent dielectric properties for bio-micro electro mechanical systems (Bio-MEMS) in sensors, actuators, and microfluidics devices.     

Chain conformation and packing of isopentylcellulose in thin films

Multilayers of isopentylcellulose (systematic name: tri-O-isopentyl cellulose, IPC) were investigated by means of electron diffraction. Only the persubstituted cellulose ether showed some order in a multilayer as deposited. IPCs of lower degree of substitution (DS) form ordered layers when annealed at temperatures ≤150°C. IPC molecules adopt helix conformation; the lattices show in this temperature range 3-fold screw axes. From analogy with known structures of cellulose derivatives we suggest left-handed 3₂-helices. At a temperature of about 210°C IPC multilayers undergo a transition into the liquid-crystalline state and the feature of a stack of individual layers disappears. This was investigated for an IPC of DS = 3. By subsequent cooling a phase transition into a highly crystalline state takes place. From the huge increase of the crystalline density from initially 1.02 g/cm³ in the multilayer system to 1.40 g/cm³ in the bulk crystal at ambient temperature after having passed the liquid-crystalline state we infer that the transition of the layered system into the liquid-crystalline state is accompanied by selection of adjacent chains according to their polarity. Simultaneously the chain conformation is changed into a 4-fold screw with 2 symmetry related chemical repeats forming the asymmetrical unit. Such a chain conformation is not known for other cellulose derivatives, only trimethylamylose adopts a similar conformation with a distorted 4₃-helix.     

Characterization of isotactic polypropylene obtained with ethylenebis(4,5,6,7-tetrahydro-1-indenyl)zirconium dichloride as catalyst

Polypropylene (PP) prepared with ethylenebis(4,5,6,7-tetrahydro-1-indenyl)zirconium dichloride in conjunction with methylaluminoxane yields 58 wt.-% of material insoluble in boiling heptane (C₇). The C₇-soluble and the C₇-insoluble fraction exhibit high isotacticity, viz. 97,2% and 98,2%, respectively, relatively narrow molecular weight distribution (weight- to number-average molecular weight M?_w/M?_n = 2,19 for the whole polymer), a small amount of regioirregular units arranged with meso-placements and relatively low melting points, i.e., 151,3°C for the C₇-soluble and 153,2°C for the C₇-insoluble fraction in spite of high isotacticity.     

Solid-state 13C NMR of microscopic order in transcrystalline films of isotactic poly(propylene)

The microscopic orientational order in a variety of transcrystalline films of isotactic poly(propylene) was investigated by means of two-dimensional solid-state NMR. From the experimental results the orientation distribution function of the microcrystallites relative to the macroscopic growth direction was determined. The NMR measurements were performed at room temperature on samples that had been crystallized at conditions at which the morphology of spherulitic crystallization changes as a function of temperature or composition. For transcrystalline samples of isotactic poly(propylene) crystallized in the α-modification a drastic breakdown of molecular order was observed at a crystallization temperature of 137°C, with the absolute degree of order depending on the molecular mass of the polymer. For samples crystallized in the β-modification a small stepwise decrease of molecular order was observed at a crystallization temperature of 128°C. A continuous decrease of the orientational order of the isotactic part was observed by blending isotactic poly(propylene) with atactic poly(propylene).     

Epitaxial crystallization of trans-1,4-polybutadiene on highly oriented isotactic poly(propylene) films

Epitaxial crystallization of trans-1,4-polybutadiene (PBD) on highly oriented isotactic poly(propylene) (iPP) has been investigated at different crystallization temperatures and rates. From electron microscopy and electron diffraction, it is confirmed that epitactic growth of the low-temperature modification (monoclinic) with microcrystals of PBD on the iPP films takes place with their chain directions about ±50° apart. No epitaxial relationship occurs between the high-temperature modification (hexagonal) of PBD and the iPP substrate. Thermal analyses of PBD-PP layered films indicate that the epitaxy has an important effect on the formation of the high-temperature modification of PBD.     

Characteristics of rectifying devices with thin films of polyphthalocyanine

Chemically and thermally stable thin films of polyphthalocyanine were prepared by a simple evaporation-polymerization method. The rectifying characteristics of metal/semiconductor/metal (MSM) type sandwich devices with the film were studied. A Schottky type device metal (Cu, Al, Ti)/polyphthalocyanine/Cu shows reproducible rectifying characteristics when Ti is selected as a counter electrode. The reproducibility is improved by pre-oxidation treatment of the surface of Cu substrate. Best electric parameters for the device are as follows: rectifying ratio = 14; threshold potential difference = 0,61 V; saturation current = 2,5 · 10^?6 A · cm^?2; barrier height = 0,75 eV; diode ideality parameter = 4,23. Doping of five kinds of quinones [p-benzoquinone (p-BQ), tetrabromo-p-benzoquinone (p-TBBQ) tetrachloro-p-benzoquinone (p-TCBQ) tetrachloro-o-benzoquinone (o-TCBQ) 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ)] and 2,5-cyclohexadien-1,4-diylidenedimalodinitrile (TNCR) in thin films of polyphthalocyanine affected electrocharacteristics in some cases. The diode ideality parameter decreases to 2,17 at 0,90 · 10^?6 mol · cm^?1 of p-TBBQ, and the rectifying ratio increases to about fourteen times by doping p-TBBQ and p-TCBQ. The doped device shows a rectifying response up to 1 kHz.