Conformations in partly ionized poly(methacrylic acid): 1. Ionic hydrogen bonds in syndiotactic chains

Comparisons of u.v. spectrophotometric titrations of some dicarboxylic acids, ‘conventional’ poly(methacrylic acid) and poly(acrylic acid) suggest that ionic or acid-salt hydrogen bonds form between adjacent carboxyl groups in racemic dyads in partly ionized forms of poly(methacrylic acid). The ionic hydrogen bonds form in tt conformations of the dyads. The maximum concentration of ionic hydrogen bonds occurs at 50% ionization in pure syndiotactic chains in the presence of electrolyte to give a chain having four-bond repeating units. Bonds 1 and 2 are presumed to be fixed in trans-trans conformations while bonds 3 and 4 retain some conformational freedom although this is somewhat reduced as a consequence of side group rotations necessary for hydrogen bonding. While ionic hydrogen bonds may form across meso dyads in t? conformations, isotactic chains incorporating ionic hydrogen bonds are predicted to be less stable than the corresponding syndiotactic chains.     

Synthesis of isotactic polystyrene in hydrocarbons by initiation with t-BuLi in the presence of sodium dodecylbenzenesulfonate

Isotactic rich polystyrene was synthesized at 30 °C by the t-BuLi initiated polymerization in the presence of sodium dodecylbenzenesulfonate (SDBS) in hexane or cyclohexane. The polymerization rates at 30 °C were very fast and typically gave quantitative conversions. At SDBS/t-BuLi molar ratios between 0.5 and 1.0, the unfractionated polystyrene had almost the same stereoregularity, with triad and pentad contents of about 45% and 19%, respectively. The polymers could be fractionated into a more isotactic PS (mm = 0.77) and other fractions with lower isotactic content. The influence on the stereochemistry of lithium and potassium dodecylbenzenesulfonates or other sulfonate derivatives, reaction temperature and solvent have also been investigated. Density functional theory (DFT) was used to simulate the reactive sites using 1-lithio-1,3-diphenylbutane and 4-methylbenzenesulfonate (SBS) were used as simplified models of polystyryllithium and SDBS respectively. DFT calculation results indicated that unlike the simpler LDPB model and LDPB-styrene complexes, the pro-m-SBS-LDPB-styrene complexes were the preferred stereochemical configuration consistent with the formation of long isotactic pentad sequences.     

The melt compatibility of blends of polypropylene and ethylene-propylene copolymers

Blends of polypropylene with ethylene-propylene copolymers of various compositions have been studied by small angle neutron scattering with regard to their compatibility at room temperature and in the melt. It has long been known that such blends separate into distinct phases at lower temperatures due to the crystallinity of the isotactic polypropylene, The work described herein has shown that these blends are also immiscible in the melt, even where the ethylene content of the copolymer is as low as 8 percent. Moreover, the separated phase domains grew rapidly at melt temperatures. Blends of atactic polypropylene with isotactic polypropylene did become miscible upon melting.     

Stereoblock polypropylene/isotactic polypropylene blends. I. Phase organization

The phase organization of stereoblock polypropylene/isotactic polypropylene blends has been analyzed. The different samples were prepared by a two-stage process, that is, solvent casting followed by pressure molding. The analysis was carried out using techniques such as X-ray diffraction, differential scanning calorimetry (DSC), and thermomicroscopy. The experimental results show that both the components segregate a crystalline phase and that the overall crystallinity, as well as the crystallinity of each component, is affected by the sample composition. Some evidence of co-crystallization, at least for samples at low content in isotactic polypropylene, has been found. © 1994 John Wiley & Sons, Inc.     

New polypropylene blends toughened by polypropylene/poly(ethylene-co-propylene) in-reactor alloy: Compositional and morphological influence on mechanical properties

A new toughening agent, polypropylene/poly(ethylene-co-propylene) in-reactor alloy (EP-P), has been adopted to modify isotactic polypropylene (PP) in present study. Systematic investigation has been performed on the inter-compositional interaction, crystalline structure, and phase morphology of a series of PP/EP-P blends. It has been found that the PP component from EP-P is thoroughly miscible with neat PP and they together serve as the matrix of the PP/EP-P blends, while the ethylene-propylene random copolymers (EPR) act as the dispersed phase. The ethylene-propylene segmented copolymers (EPS), behaving as the compatibilizer between the EPR dispersed phase and PP matrix, strengthen mutual incorporation and effective diffusion of the amorphous PP segments and the EPR molecules. Based on the in-depth understanding of the crystalline structure and phase morphology, the correlation between morphological structure and mechanical properties has been established. The excellent impact toughness of PP/EP-P blends with higher EP-P content is mainly attributed to the small PP crystallites scattered in the blends and the well dispersed EP copolymer domains in PP matrix.     

High pressure crystallization of random propylene-ethylene copolymers: α-γ Phase diagram

Two random propylene copolymers with low ethylene content synthesized by Ziegler-Natta catalysts were used is this study to investigate the formation of γ-crystal phase during isothermal crystallization at high pressures. At atmospheric pressure these copolymers crystallize in a mixture of α- and γ-crystals. The content of the γ-phase in the copolymer crystals increased with increasing defect content, crystallization temperature and pressure. Wide-angle X-ray diffraction studies showed that crystallization of these copolymers at pressures above 88 MPa and temperature above 142 °C leads to formation of pure γ-phase. The equilibrium melting temperature of the γ-phase has been determined as a function of defect content and crystallization pressure. Temperature-pressure-composition α-γ phase diagram of isotactic polypropylene was constructed based on the Gibbs free energy approach. This diagram enabled the extrapolation of the equilibrium melting temperatures of both phases for defect free isotactic polypropylene. They were found to be 186.9°C for the α-phase and 189.9°C for the γ-phase.     

Tacticity dependence of excimer formation in polystyrene solutions

Summary This work deals with the excimer formation in dilute solution for a series of polystyrenes of various tacticity prepared by epimerization of an initial isotactic sample. The main result is that the relative excimer yield remains equal to that obtained for iPS if the percentage of isotactic dyads is higher than 65%.The range of energy migration could be roughly approximated to 6 chromophores.     

Polypropylene Blends with m-EPR Copolymers: Structure,Morphology and Thermal Properties

The effects of different contents of two metallocene propylene-based m-EPR elastomers on structure, morphology, thermal, and dynamic mechanical properties of the isotactic polypropylene/m-EPR blends were investigated. The both m-EPR copolymers have been built in isotactic polypropylene matrix as amorphous phase. However, the nucleation effect at lowest addition (2.5?vol%) and the solidification effect along with increased m-EPR’s additions have caused changes of the crystallinity degree and the size of spherulites in the isotactic polypropylene matrix. Higher degree of miscibility/compatibility of the isotactic polypropylene/m-EPR2, with lower viscosity has been observed. Homogeneous dispersion of m-EPR particles as well as their radial distribution has been observed.     

Crystallization properties of elastomeric polypropylene from alumina-supported tetraalkyl zirconium catalysts

An analysis of the crystallization properties of fractions of elastomeric polypropylene (ELPP) prepared with Al₂O₃-supported tetraalkyl zirconium catalyst is presented. A comparison with the polymorphic behavior of isotactic polypropylene (iPP) samples prepared with a single center homogeneous metallocene catalyst is also shown. The ELPP sample has been fractionated by extraction with boiling solvents. The irregular fraction insoluble in pentane and soluble in hexane crystallizes from the melt almost totally in the γ form, whereas the more stereoregular fraction, insoluble in n-heptane, crystallizes mainly in the α form. The relative amount of γ form crystallized from the melt is much lower than the one formed in samples of metallocene-made iPP samples with a similar average content of isotactic stereosequences. Since the γ form crystallizes in chains having short regular isotactic sequences, these data indicate that in the fractions of the ELPP sample the regular isotactic sequences are longer than those present in chains of metallocene-made iPP. In particular, in the more irregular crystalline fractions of the ELPP sample the chains are characterized by a stereo-block microstructure, consisting in the presence of quite regular isotactic sequences alternating with irregular sequences, the latter containing most of the defects. The presence of stereoblocks allows crystallization of these highly irregular fractions, even in the presence of a very high content of defects, and accounts for the elastomeric properties shown by this sample. The whole ELPP sample is constituted mostly by amorphous ether soluble (40%) and pentane soluble (26%) fractions and shows elastic properties due to the high molecular weight of chains in all the fractions and the crystallization of isotactic sequences present in the stereoblocks.     

A study of the CCl stretching region of the Raman spectrum of PVC

The C-Cl stretching regions of the Raman spectra of a number of samples of PVC prepared by various methods which lead to differences in tacticity, conformational content and crystallinity have been studied. It is shown that the spectra of all samples can be fitted well with a set of nine Lorentzian peaks whose positions and widths are the same for each spectrum. Two of these peaks are attributable to the A_g and B_3g species vibrations of crystalline material and the remaining seven may be assigned with reasonable confidence to specific structures in the amorphous regions, three of which are associated with isotactic placements. The area under the A_g and B_3g peaks may be used to estimate the crystallinity of the samples and the area under the peaks assigned to isotactic material may be used to estimate the tacticity of the polymer.     

Gelation of attractive particles in polymer melt

The physical gelation of the attractive carbon black particles in the isotactic polypropylene melt is studied. Following the method proposed by Winter and Chambon, oscillatory rheology is used to study the kinetic gelation process. The gel time, the relaxation exponent of the critical gel, the gel stiffness and the activation energy for the gelation are determined. The influences of the filler concentration and the experimental temperature on the gelation process are also investigated. The results suggest that the gelation of the filler particles in the polymer melt mainly follows second-order kinetics, and the critical gel should be formed by the rigid particles and the inter-particle polymer layers.     

Synergistic gelation of solutions of isotactic polypropylene and bis-(3,4-dimethyl benzylidene) sorbitol and its use in gel-processing

We report the synergistic, rapid gelation of solutions of isotactic polypropylene (i-PP) and the nucleating agent 1,3:2,4-bis-(3,4-dimethyl benzylidene) sorbitol (DMDBS) in decalin. Cooling to room temperature of a solution comprising, for instance, 3.0 wt% of a moderately high molecular weight i-PP (M_v=1.3×10⁶ g/mol) and as little as 0.0075 wt% DMDBS (0.25 wt% based on the polymer) resulted in the fast formation of highly ductile gels. In reference experiments without DMDBS, often ‘mud-cracked’, brittle polymer films were obtained, and decaline solutions of DMDBS alone at the above concentration did not form macroscopically coherent gels. In the present work we employed this useful occurrence for gel-processing/drawing of i-PP, yielding material with Young's moduli of 35 GPa, tensile strengths of approximately 1 GPa and melting temperatures measured for constrained samples as high as 228 °C.     

Mechanical Properties of Impact i-PP/CSM Rubber Blends

ABSTRACT

The impact strength and the tensile behavior of an impact grade of isotactic polypropylene (impact i-pp)/chlorosulfonated polyethylene (CSM) rubber blends are studied at the CSM rubber concentrations 0 to 23.4 vol%. The impact strength, which increased with CSM rubber concentration, has been analyzed on the basis of the interphase adhesion and crazing mechanisms. Tensile modulus and strength decreased whereas breaking elongation increased with increase in CSM rubber content. Predictive models have been used to explain the tensile modulus and strength properties. Scanning electron microscopy has been employed to study the phase structure.     

Residual solvent removal and n-hexane sorption in blends of atactic and isotactic polystyrene

Atactic polystyrene, isotactic polystyrene, and intermediate polyblend films of the homopolymers were cast from solutions of 1,3,5-trimethylbenzene (mesitylene) or o-chlorotoluene. Residual solvent content, glass transition temperatures, and densities of the polyblends were determined. Consequent to these preliminary characterizations, n-hexane absorption kinetics and equilibria in unannealed blends were determined; maxima in the plots of weight change versus time were apparent for all compositions. The maximum is a consequence of the ‘unlocking’ of previously trapped residual casting solvent consequent to plasticizing invasion by the n-hexane in the unannealed film specimens. Neutron activation analysis for chlorine confirmed that the o-chlorotoluene content in the films was reduced dramatically consequent to n-hexane absorption. Residual solvent content was also reduced by thermally annealing the films at 110°C for 48 h under a vacuum and, therefore, no maximum in the sorption/time behaviour was observed in thermally annealed samples nor in films previously contacted with n-hexane vapour and subsequently vacuum stripped. Both the rate of sorption and the apparent sorption equilibrium are greatly reduced as the isotactic polystyrene content is increased. Predominantly Case II or relaxation controlled absorption kinetics were observed in the polyblends consequent to thermal annealing and/or ‘solvent’ annealing. Although the form of the sorption kinetics was quite similar for absorption of n-hexane in thermally annealed or ‘solvent-annealed’ films, the sorption rate and amount of penetrant sorbed at apparent equilibrium were both larger for the thermally annealed films. These results suggest that the history dependence, which so frequently affects polymeric glasses, appears to confound a simple interpretation of the sorption experiments reported here. Consistent with observations in related systems, Super Case II kinetics were observed for the more slowly sorbing films which, in this case, contained isotactic polystyrene. Moreover, film whitening related to micro-voiding accompanied n-hexane sorption in blends containing atactic polymer; however, the whitening in the isotactic polymer was less pronounced and densification, presumably related to solvent induced crystallization, occurred consequent to long term exposure of the isotactic polymer to n-hexane.     

Preparation and properties of stereoregular poly(hydroxyethyl methacrylate) polymers and hydrogels

Linear poly(hydroxyethyl methacrylate) (PHEMA) has been synthesized in highly syndiotactic and highly isotactic configurations. The high syndiotactic PHEMA prepared by u.v. photolysis at ?40° C was found by ¹³C n.m.r. to have a tactic triad content of 84% syndio, 16% hetero and 0% iso. High isotactic PHEMA was prepared by anionic polymerization of benzoxyethyl methacrylate in toluene followed by selective hydrolysis of the benzoate ester, and was observed by ¹³C n.m.r. to have a 5% syndio, 15% hetero and 80% isotactic triad content. A linear PHEMA polymer formed by radical polymerization at 60°C in ethanol solvent was found to have a tactic triad content of 58% syndio, 42% hetero and 0% iso. These polymers have been crosslinked with hexamethylene diisocyanate and their water swelling properties determined as functions of temperature and crosslinker concentration. Isotactic PHEMA exhibited greater aqueous swelling below 30° C than the syndiotactic PHEMA samples. The stereochemistry of the polymer chain is shown to be a factor in determining the swelling behaviour of hydrophilic methacrylate gels.     

Thermoreversible gelation of biodegradable polyester (PHBV) in toluene

In present paper we investigate thermoreversible gelation of biodegradable polyester poly (3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) in toluene. Hot PHBV solutions became transparent gels after cooling to room temperature. This physical gelation process was followed by light scattering and viscosity measurements for solutions of different PHBV concentrations. It has been found that gelation temperature increases with increasing polymer concentration in toluene. PHBV films have been prepared by gelation process followed by solvent removal on solid substrates. It has been demonstrated that PHBV concentration in the solution influences the surface morphology of obtained films. Homogeneous PHBV films with increased surface roughness can be obtained by means of developed technique. Hydrolytic degradation studies indicate that surface morphology of obtained PHBV layers changes considerably with degradation time.     

1H nuclear magnetic relaxation in isotactic poly(methyl methacrylate) in dilute solution

¹H spin-lattice and spin-spin relaxation times have been measured for individual groups of nuclei in isotactic poly(methyl methacrylate) in dilute solution in toluene-d₈. The relaxation times have been analysed in terms of backbone conformational transitions and methyl rotation.     

Percolation of glassy regions during photopolymerization of epoxy acrylate in DMF

The gelation of epoxy acrylate (EA) 80% and tripropyleneglycol diacrylate (TPGDA) 20% was studied through the use of photo-differential scanning calorimetric (photo-DSC) technique in the presence of a thioxanthone based initiator. Photo-induced polymerization reactions were performed under identical conditions of temperature, initiator concentration and UV light intensity in various N,N-dimethylformamide (DMF) contents. Photo-DSC technique allowed us to monitor the gelation without disturbing the system mechanically, and to test the universality of the gelation as a function of DMF content. During gelation, it was observed that all conversion curves presented useful sigmoidal behavior which was predicted by employing a percolation model. Observations around the glass transition point, t_g, revealed that gel fraction exponent β obeyed the percolation picture. A significant solvent effect on the photopolymerization kinetics of EA/TPGDA was observed with changes in DMF content. DMF is used as solvent, which acts as a diluent and proton donor during photogelation.     

Effects of cononsolvency on gelation of poly(vinyl alcohol) in mixed solvents of dimethyl sulfoxide and water

We studied the rates of gelation and phase separation of poly(vinyl alcohol) (PVA) solutions in mixtures of dimethyl sulfoxide (DMSO) and water at 25 °C and found that both the rates show a maximum at a volume fraction of DMSO φ_DMSO=0.60 while gelation was not observed either in pure DMSO or pure water, suggesting that water-DMSO is a cononsolvent system for PVA. On the basis of the data by Cowie [Can J Chem 36 (1961) 2240] we concluded that the 1:2 stable complex between one DMSO molecule and two water molecules is the main cause of this cononsolvency.     

Stereoblock polypropylene/isotactic polypropylene blends. II. Mechanical behavior

The mechanical properties of stereoblock polypropylene/isotactic polypropylene blends have been analyzed at different temperatures and at large deformations. The samples at a lower content of isotactic polypropylene show good elastic properties, both in terms of deformation reversibility and in terms of energy dissipation in the hystersis cycles. Considering the dependence of the elastic behavior on the temperature, the model that can be suggested is based on the presence of a physical network in which the cross-linking is due to cocrystallization between the matrix (the stereblock polypropylene) and the crystalline domains of isotactic polypropylene. © 1995 John Wiley & Sons, Inc.