Modification of isotactic poly(propylene) by oxygen and helium plasma with reference to thermo-oxidative stability

The modification of isotactic poly(propylene) (PP) foils by reactive (O₂) and inert (He) gas plasma treatment was studied with relevance to surface structure and thermo-oxidative stability for different treatment times. The following effects were investigated: change of non-helical (atactic or amorphous) content in the surface region and formation of vinyl and vinylidene groups by frustrated multiple internal reflection Fourier transform infrared spectroscopy (FMIR-FTIR), formation of peroxyl redicals by electron spin resonance spectroscopy (ESR) and change of the invariant activation energy of degradation by thermogravimetric analysis (TGA). For the latter a new kinetic evaluation method was used based on a selection of multiple step model to determine the invariant Arrhenius constants, e.g., activation energy and preexponential factor, for the weight loss during thermoxidative degradation of untreated and, for the first time, of plasma-treated PP. Several thermogravimetric curves under different temperature scanning conditions have been evaluated with different solid state models as well as with a selection of different multiple step reaction pathways. With a limitation to total weight loss of 60% of the mass for an untreated PP and applying a most probable kinetic model consisting of one consecutive and one parallel reaction step, the activation energies were found to be 84.2 ± 1.0 and 73.2 ± 1.0 kJ/mol for the initial and the consecutive reaction and 137.8 ± 3.0 kJ/mol for the parallel reaction. The corresponding data of the plasma-treated samples are 80 ± 1.0 and 71.0 ± 1.0 kJ/mol for the initial and the consecutive reaction, while the activation energy for the parallel reaction remained almost unchanged. Unexpectedly, we found the He plasma treatment to decrease the activation energies of the first and consecutive reaction nonlinearly and within the first 600 s to a larger extent than O₂ plasma treatment. Based on this kinetic model, a modified mechanism for the thermo-oxidative degradation of PP is proposed in terms of elementary degradation reactions and will be discussed with reference to literature data.     

On the stereoisomerization in thermal degradation of isotactic poly(propylene)

The terminal microstructure of the hydrogenated α,ω-diisopropenyloligo(propylene) prepared by thermal degradation of isotactic poly(propylene) at 370°C was determined by means of ¹³C NMR spectroscopy, and therefrom the stereoisomerization reaction of the reacting polymer segment is discussed in relation to polymer dynamics. With respect to the asterisked carbons of the propyl end group CH₃CH₂^*CH₂ (n-Pr) and of the isopropyl end group (^*CH₃)₂CH (i-Pr) formed by hydrogenation, the mole fractions of triads (m meso, r racemo) F_mm = 0.64, F_mr = 0.09, F_rr = 0.12 of n-Pr-^*CH₂ differ from the microtacticity of the main chain of telechelic oligomers, and the mole fraction of i-Pr-^*CH₃ is as follows: F(δ_tζ_t) + F(δ_eζ_e) = 0.74 (m); F(δ_tζ_e) + F(δ_eζ_t) = 0.26 (r). These results clearly show that the stereoisomerization occurs only at the asymmetric carbon near the end group of the telechelic oligomer. The value of the mole fraction of i-Pr-^*CH₃ is consistent with F_m = 0.73 (F_mm + F_mr) and F_r = 0.27 (F_rr + F_rm) of n-Pr-^*CH₂ at the outer position of the main chain rather than with F_m = 0.79 (F_mm + F_rm) and F_r = 0.21 (F_rr + F_mr) at the inner position. These results could be consistently traced by a simulation based on a reaction model including stepwise intramolecular hydrogen abstraction of the tertiary on-chain macroradical near the center of the main chain before the successive β-scission. This stepwise back-biting is estimated to occur once (20%) and twice (80%) via a quasi six-membered cyclic structure in a transition state, and occurs in a very limited part of the molecular chain.     

The crystal structure of isotactic poly (propylene oxide)

The results of a detailed investigation on the crystal structure of poly(propylene oxide) are reported. The chain model here proposed differs from the full-planar one only by a deviation of 15° around the C? O bonds. This model is accounted for by both the sensibly better agreement between F_o and F_c and the more justifiable intra-chain contacts with respect to those observed for the model previously studied by STANLEY and LITT , with which it is compared.     

Crystallization of the γ-form of isotactic poly(propylene)

The pure γ-form of isotactic poly(propylene) can be obtained by melt-crystallization starting from a fraction of a random ethylene-propene copolymer (ethylene content 4 wt.-%) soluble in p-xylene at 28°C. Some structural models previously reported are tested by comparing calculated wide-angle X-ray diffraction patterns with the experimental ones. A method for the quantitative determination of the γ-form crystallinity by X-ray line profiles is proposed. The original copolymer and its fractions are characterized by differential scanning calorimetry and ¹³C NMR techniques.     

Modification of poly(propylene) through grafting with dimethyl itaconate in solution

Functionalization of isotactic poly(propylene) (PP) with dimethyl itaconate (DMI) as functional polar monomer using 2,5-dimethyl-2,5-bis(tert-butylperoxy)hexane (L101) as radical initiator was carried out in both boiling xylene and decalin as solvent media. The effect of DMI and the initiator concentration on the extent of grafting was studied by varying reaction time and temperature. It was found that temperature affects the percentage of DMI grafted onto PP, which is slightly higher for reactions carried out in xylene than in decalin. The results also show that the amount of DMI incorporated is proportional to the initial DMI and initiator concentrations used in the grafting reaction up to certain concentrations, and thereafter a decrease in the percentage of grafting (G in wt.-%) was found. The maximum value of grafting obtained was 0.7 wt.-%. The melt flow index (MFI) values increase with increasing initial amount of initiator used in the grafting reaction. The degradation of the PP chain is higher when xylene is used as solvent. MFI values of 20–100 were found for modified PP compared with 11.4 for the unmodified polymer.     

Copolymerization of carbon dioxide with propylene oxide catalyzed by poly(p-hydroxystyrene)/diethylzinc system

A polymeric catalyst prepared from poly[1-(4-hydroxyphenyl)etylene] ( 1 ), and Zn(C₂H₅)₂ was found to be effective for the copolymerization of carbon dioxide and 1,2-epoxypropane. The highest activity was observed when a mole ratio of phenolic hydrogen in 1 to Zn(C₂H₅)₂ equal to unity was applied in contrast to the catalysts so far reported. The structure of the catalyst after the copolymerization was determined by IR and H¹ NMR spectroscopy. A Zn—C₂H₅ bond in this system was confirmed to be the active center. A possible inactivation process of the catalyst is proposed.     

Nucleation and crystallization of isotactic poly(propylene) droplets in an immiscible polystyrene matrix

In this work, isotactic poly(propylene) (iPP) was finely dispersed in immiscible atactic polystyrene (PS) matrices. When the dispersion obtained is fine enough (droplet size of approximately 1–2 μm), the iPP crystallizes in a fractionated fashion at temperatures between 104 and 42°C. By applying a self‐nucleation procedure we were able to corroborate that what causes the fractionated crystallization in most droplets is the lack of highly active heterogeneous nuclei (i. e. those normally active at low supercoolings in the bulk polymer) in every droplet. When a sufficient amount of a compatibilizer is used to obtain very small particle sizes and more homogeneous dispersions, the iPP crystallizes exclusively at a low temperature exotherm that exhibits an onset at 51°C and peaks at 46°C. Wide‐angle X‐ray diffraction measurements indicated that both the iPP in the bulk and in dispersed droplets crystallized in the monoclinic α‐phase, this evidence may rule out the possibility that the crystallization observed at 46°C is due to the formation of another crystal modification or a mesomorphic phase as previously suggested in literature. The results presented in this work indicate that this low temperature exotherm may represent the dynamic crystallization during cooling of heterogeneity‐free droplets that nucleate homogeneously at temperatures close to 51°C.     

SAXS/WAXS study of the annealing process in quenched samples of isotactic poly(propylene)

The structural rearrangement in samples of quenched isotactic poly(propylene) (iPP) submitted to different annealing treatments has been studied using simultaneous small- and wide-angle X-ray scattering (SAXS/WAXS) at the synchrotron radiation source of DESY, Hamburg. From a quantitative analysis of the WAXS profiles the values of the α-monoclinic, mesomorphic, and amorphous mass fractions coexisting in the material were determined. It is demonstrated that the SAXS patterns were characterized by two different long-period values that are attributed to α- and mesomorphic periodicity, respectively. The related α- and mesomorphic volume phase fractions, calculated from the analysis of the SAXS data, are compared with the corresponding WAXS results. The rearrangement of the initial structure involves i) the thickening of the already existing α-phase lamellae at the expenses of the amorphous regions and ii) the structural rearrangement of the mesomorphic phase leading to its transformation into the α-monoclinic one.     

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.     

Dispersion of trans-polyoctenylene in isotactic poly(propylene), 2. Crystallization behaviour

The crystallization kinetics of thin films of isotactic poly(propylene) (iPP) and transpolyoctenylene rubber (TOR) was investigated by optical microscopy. Nucleation densities, Avrami exponents and spherulitic growth rates show specific dependencies on the concentration of TOR in the blend. From the results it is concluded, that at a mass fraction of TOR w_TOR = 0,10 the TOR is more finely dispersed in the blend than for other compositions. Due to the subsequent formation of interfaces, the nucleation is found to be predominantly heterogeneous at w_TOR = 0,10, while it is preferentially homogeneous at all other compositions.     

Influence of the stretching rate on the transition front structure of uniaxially deformed isotactic poly(propylene)

Recent models on the melting process of polymer crystals allow the study of semicrystalline polymer structures by calorimetric (DSC) measurements. In particular, this work is devoted to the study of those changes occurring during the deformation process of isotactic poly(propylene) samples along the transition front. By observing the evolution of the parameter of intrachain melting cooperativity in comparison with lamellar thickness, it has been possible to characterise the birth and further evolution of double fold molecules, totally extended polymer chains connecting two adjacent crystallites in the fibrillar structure, “taut tie molecules”. Furthermore, those structural changes depend on the stretching conditions. In general, low rate promotes high lamellar fragmentation and the presence of a small number of taut tie molecules. On the contrary, at higher rates crystallites coming from the original lamella show bigger size and a higher number of tie molecules are achieved.     

Solid-state NMR studies of structure and dynamics of erucamide/isotactic poly(propylene) blends

Solid-state NMR spin diffusion experiments and two-dimensional (2D) ¹H-¹³C NMR experiments have been used to study the phase structure, mobility and domain sizes of blends of isotactic poly(propylene) (i-PP) and erucamide (ERU), a low molecular weight slip and antiblock additive for i-PP films. The study of the proton line shapes in pure erucamide and i-PP together with their blends reveals the mobility of the two components. The mobility is higher in the blends than in erucamide and i-PP when they are considered separately. In addition, 2D wideline separation (WISE) NMR spectroscopy indicates that erucamide molecules show both low and high mobilities depending on the erucamide content in the blend. By measuring the proton spin diffusion with ¹H detection, the microdomain dimensions have been estimated quantitatively. The domain sizes range from approximately 13 nm to 60 nm.     

Copolymerization of carbon dioxide with propylene oxide catalyzed by O-methylated or crosslinked poly(p-hydroxystyrene)/diethylzinc system

Effective catalysts for the copolymerization of CO₂ with 1,2-epoxypropane were prepared from partially O-methylated or crosslinked poly[1-(4-hydroxyphenyl)ethylene] and Zn(C₂H₅)₂. The highest activity was obtained with catalysts of 30 to 40% methylation or crosslinking. Inactivation of the catalysts during the copolymerization was decreased by crosslinking, increasing the yield of the copolymer. The higher rigidity of the catalyst structure is responsible for its higher activity and stability.     

Melting and crystallization behavior of syndiotactic poly(propylene) homopolymer mixtures with atactic poly(propylene)

The melting and crystallization behavior of syndiotactic (sPP) poly(propylene) homopolymer mixtures with atactic (aPP) poly(propylene) are investigated by differential scanning calorimetry (DSC) and wide angle X-ray scattering (WAXS). The tacticity microstructure of sPP homopolymer polymerized with metallocene catalysts varies with polymerization temperature. The resultant sPP microstructure is strongly coupled to the DSC melting behavior. Multiple endotherms are observed which are sensitive to heating rate, consistent with a melting-recrystallization-remelting (MRM) mechanism as a contributing factor to the higher temperature endotherm (Endotherm II). Reduced sPP tacticity and molecular weight promote the reorganization. The multiple melting response is a useful probe of aPP “diluent” segregation within the sPP/aPP blend morphology. The addition of aPP leads to a suppression of crystallization temperature, and promotion of Endotherm II on heating which is sensitive to both the “host” sPP and “diluent” aPP molecular weight. Both behaviors suggest a degree of “local” mixing of diluent within the morphology.     

Reactivity of surface groups formed onto a plasma treated poly(propylene) film

Cold plasma treatments of polymers, dry processes, allow either the elaboration of hydrophilic or hydrophobic surfaces. For example, a poly(propylene) film treated in nitrogen plasma shows a surface having a hydrophilic and basic character since amino groups are attached onto the surface during the treatment. The treatment induces an increase of the surface tension of the polymeric material, which may be sometimes destroyed by an aging effect. For the treatment of poly(propylene) in nitrogen plasma, the aging is mostly due to a motion of attached groups from the surface to the bulk of the material and some oxidation of radicals formed during plasma treatment. The surface radicals formed and used for a post-reaction such as grafting are characterized in terms of chemical nature, density and reactivity.     

Synthesis of vinyl-terminated isotactic poly(propylene) using the coupling reaction between Zn-terminated polymer and allyl halides

Propene was polymerized with the TiCl₃/Al(C₂H₅)₂Cl catalyst system using Zn(C₂H₅)₂ as a chain transfer reagent. The resulting Zn-terminated poly(propylene) (PP) was then brought into contact with allyl halides (chloride, bromide and iodide) in the presence of N-methylimidazole to obtain vinyl-terminated PP. Allyl bromide gave the highest conversion among these allyl halides. When the coupling reaction was conducted with allyl bromide at 40°C for 24 h, the conversion to the terminal vinyl groups was over 90% for atactic PP and approximately 60% for isotactic PP. The Zn-polymer bonds of both atactic and isotactic PP were almost quantitatively converted into terminal vinyl groups at 130°C for 3 h.     

Critical epitaxial layers of different kinds of polyethylene on highly oriented isotactic poly(propylene) substrates

Epitaxial crystallization of high-density polyethylene (HDPE), linear low-density polyethylene (LLDPE) and low-density polyethylene (LDPE) in the epitaxial system with highly oriented isotactic poly(propylene) (iPP) has been investigated by electron microscopy. The results obtained by bright field electron microscopy and electron diffraction confirmed that all of the three kinds of PE (HDPE, LLDPE and LDPE) have the ability of epitaxy on iPP surfaces. There exists a critical thickness of the PE layer in which the PE can grow epitaxially on iPP substrates. When the PE/iPP layered films are heat-treated at a temperature above the melting temperature (T_m) of PE and below T_m of iPP for 10 min and then quenched to room temperature, the critical layer thickness of epitaxial HDPE, LLDPE and LDPE is about 250, 120 and 30 nm, respectively. For PE further apart from the interface, the epitaxial orientation becomes disturbed due to simultaneous spherulitic nucleation or lamellar twisting of the epitaxially grown lamellae.     

Crystallization of the γ-form of isotactic poly(propylene), 2. Synchrotron radiation measurements

Synchrotron radiation measurements were employed in recording wide- and small-angle X-ray diffraction patterns during the crystallization of isotactic poly(propylene) in γ-form. In particular, crystallization phenomena of this form are reported for a fraction of a random propylene-ethylene copolymer (4 wt.-% of ethylene) previously investigated by traditional diffraction methods. On slow cooling from the melt, the main decrease in long period L occurs between 88 and 76°C where the degree of crystallinity is almost constant. This may be explained by a flattening of lamellae.     

Effect of polymerization temperature on the microtacticity of isotactic poly(propylene) prepared using heterogeneous (MgCl2-supported) Ziegler-Natta catalysts

A study of the effect of polymerization temperature on the microtacticity of poly(propylene) prepared using various MgCl₂-supported catalysts has shown that, in almost all cases, an increase in temperature leads not only to higher proportions of isotactic polymer but also to increased stereoregularity of the isotactic fraction. The results indicate a greater relative increase in productivity, with increasing temperature, for highly isospecific as opposed to moderately isospecific centres. It is suggested that easier propagation after the occasional regioirregular (2,1-) insertion may contribute to this effect.