Search

Highly accurate high-throughput experimentation (HTE) data for a set of 21 silicon-bridged C2-symmetric ansa-zirconocenes in propene homopolymerization were collected and were used to develop quant...

Plant-derived essential oils (EOs) represent a green alternative to conventional antimicrobial agents in food preservation. Due to their volatility and instability, their application is dependent on the development of efficient encapsulation strategies allowing their protection and release control. Encapsulation in Polyhydroxyalkanoate (PHA)-based nanoparticles (NPs) addresses this challenge, providing a biodegradable and biobased material whose delivery properties can be tuned by varying polymer composition. In this work, EO from Mexican oregano was efficiently encapsulated in Polyhydroxybutyrate (PHB) and Poly-3-hydroxybutyrate-co-hydroxyhexanoate (PHB-HHx)-based NPs by solvent evaporation technique achieving high encapsulation efficiency, (>60%) and loading capacity, (about 50%). The obtained NPs displayed a regular distribution with a size range of 150–210 nm. In vitro release studies in food simulant media were fitted with the Korsmeyer–Peppas model, indicating diffusion as the main factor controlling the release. The cumulative release was affected by the polymer composition, possibly related to the more amorphous nature of the copolymer, as confirmed by WAXS and DSC analyses. Both the EO-loaded nanosystems displayed antimicrobial activity against Micrococcus luteus, with PHB-HHx-based NPs being even more effective than the pure EO. The results open the way to the effective exploitation of the developed nanosystems in active packaging.

Isotactic polypropylene-block-polyethylene block copolymers (iPP-b-PE) composed of crystalline blocks of isotactic polypropylene (iPP) and linear polyethylene (PE) of different block lengths have been synthesized with a living and stereoselective pyridylamidohafnium catalyst. The structure and morphology of iPP-b-PE that develop upon crystallization from the melt are presented. Temperature and time resolved X-ray diffraction experiments in wide (WAXS) and small angles (SAXS) recorded during cooling from the melt indicate that PE blocks crystallize first upon cooling and the iPP block crystallizes at lower temperatures and nucleates over the crystals of PE formed before. The SAXS data indicate that PE lamellar stacks formed first at high temperature have higher periodicity of 30 nm, whereas iPP lamellar stacks formed after at lower temperature have lower periodicity of 11–12 nm. The crystallization of the iPP and PE blocks drives microphase separation and determines the final morphology leading to a lamellar morphology characterized by alternation of amorphous and crystalline layers. At nanoscale the morphology is characterized by alternating iPP and PE microdomains, each domain including stacks of lamellar crystals of iPP or PE of different periodicities, separated by the corresponding amorphous regions of iPP or PE. Polarized optical micrographs of the melt-crystallized samples show the presence of bundle-like crystals in samples with longer block of iPP and of small banded spherulites in samples with longer block of PE. The observation of banded spherulites typical of PE suggests that the crystallization of PE defines the global morphology and is not altered by the structure of the melt, confirming that crystallization of PE occurs first upon cooling from the melt.

Ethylene (E)-co-norbornene (N) copolymerizations were conducted by using two ansa-metallocenes, isopropyliden(eta(5)-cyclopentadienyl)(eta(5)-indenyl)zirconium dichloride (1) and isopropyliden(eta(5)-3-methylcyclopentadienyl)(eta(5)-fluorenyl)zirconium dichloride (2), activated with dimethylanilinium tetrakis(pentafluorophenyl)borate, in presence of TIBA and variable amounts of Zn(Et)(2) as a potential chain transfer agent (CTA). The study evaluated the effect of CTA on microstructure, molecular weight, chain end group and structural, thermal and mechanical properties of poly(E-co-N)s. The analysis of the data revealed that in all copolymerization reactions with metallocenes 1 and 2 in presence of diethyl zinc chain transfer to zinc alkyl occurs along with the typical chain transfers of E-co-N polymerization. The chain transfer to zinc alkyl is more efficient at low N content in copolymers. The two catalysts show different behavior in copolymerization at high N content: molar masses of copolymers produced by 2 are more affected by CTA concentration, that is, the chain shortening with increasing CTA content is greater in copolymers obtained with catalyst 2. The analysis of mechanical properties revealed that CTA affects the polymer chain structure. An increase of CTA caused a change of ductility, rigidity, and mechanical strength; chains produced in presence of Zn(Et)(2) are, at least in part, characterized by segments produced as a result of exchange between different metal centers, possibly jointed through ethylene units.

The structure and morphology of samples of syndiotactic polypropylene (sPP) having different stereoregularity with concentration of rrrr pentads from 78 to 95.8%, obtained by quenching the melt to 0°C, is analyzed by wide angle X-ray scattering (WAXS) and atomic force microscopy (AFM). The fast cooling from the melt induces the formation of nanocrystals in the trans-planar mesomorphic form of sPP and results in a morphology characterized by the presence of globular entities (nodules) and no higher-order superstructure. It is shown that the nodule sizes depend on the stereoregularity of the sPP samples and the permanence time at 0°C. The observed nodular morphology is completely different from the lamellar morphology typically obtained in sPP samples melt crystallized at lower cooling rate. This result indicates that crystals of different shape, size and higher-order organization can be generated by changing the crystallization pathway.

Chronic respiratory infections are the main cause of morbidity and mortality in cystic fibrosis (CF) patients, and are characterized by the development of multidrug resistance (MDR) phenotype and biofilm formation, generally recalcitrant to treatment with conventional antibiotics. Hence, novel effective strategies are urgently needed. Antimicrobial peptides represent new promising therapeutic agents. Here, we analyze for the first time the efficacy of three versions of a cryptide identified in human apolipoprotein B (ApoB, residues 887-922) towards bacterial strains clinically isolated from CF patients. Antimicrobial and anti-biofilm properties of ApoB-derived cryptides have been analyzed by broth microdilution assays, crystal violet assays, confocal laser scanning microscopy and scanning electron microscopy. Cell proliferation assays have been performed to test cryptide effects on human host cells. ApoB-derived cryptides have been found to be endowed with significant antimicrobial and anti-biofilm properties towards Pseudomonas and Burkholderia strains clinically isolated from CF patients. Peptides have been also found to be able to act in combination with the antibiotic ciprofloxacin, and they are harmless when tested on human bronchial epithelial mesothelial cells. These findings open interesting perspectives to cryptide applicability in the treatment of chronic lung infections associated with CF disease.

The correlations between the thermal behavior and the crystal morphology of isotactic propene-pentene copolymers were studied through wide-angle (WAXS) and small-angle (SAXS) X-ray diffraction. Copolymers with pentene concentration lower than 11 mol% crystallize in the α form of isotactic polypropylene (iPP) and a concomitant decrease of melting temperature and of the thickness of crystalline lamellae with increasing pentene concentration has been observed. At higher pentene concentrations the trigonal form of iPP crystallizes and a neat increase crystalline lamellar thickness and of the long period, with a slower decrease of melting temperature and crystallinity have been observed. These results have been treated in the general framework of copolymer crystallization theories, using a method proposed by Crist and correlated with the different level of inclusion of pentene co-units in the crystals of α and trigonal forms. For copolymers with pentene concentration lower than 11 mol% pentene co-units are in part incorporated in the crystals of α form. For higher pentene concentrations the decrease of melting temperature coupled with the increase of lamellae thickness with increasing comonomer content is the hallmark of the almost total inclusion of pentene co-units in the crystals of the trigonal form.

The mechanical behavior of propylene−pentene isotactic copolymers synthesized with a single-center organometallic catalyst in a broad interval of pentene (Pe) concentration, from 3 to 50 mol %, is analyzed. All copolymers show flexible behavior with very high deformation at breaking. The presence and entrance of Pe counits in α and δ forms of isotactic polypropylene (iPP) transform a strong and fragile iPP homopolymer into ductile and flexible materials. The stress at yield decreases as the Pe content increases, but a different behavior has been observed for samples in the α form or in the δ form. In samples in the α form, from 3 to 8.8 mol % of Pe, the stress at yield increases as the thickness of crystalline lamellae increases, while in samples in the δ form containing higher Pe counits, it decreases as the thickness of lamellae increases and Pe content increases. Phase transition of the α form in the mesophase and crystallization of the δ form, in samples with 8−11 mol % of Pe, occur upon deformation and are correlated to the strain-hardening observed at high strain.

The crystallographic micromechanical model (CMM) for prediction of yield stress of semicrystalline polymers, based on the thermally activated nucleation of screw dislocations at the boundary of lamellar crystals, is employed to interpret the yield behavior by effect of uniaxial drawing of some isotactic copolymers of propylene with different comonomeric units such as ethylene, 1-butene, 1-pentene, 1-hexene, and 1-octadecene (iPPEt, iPPBu, iPPPe, iPPHe and iPPOc, respectively). The samples are characterized by a random distribution of the comonomeric units. The CMM predicts that the values of stress at yield depend on the thickness of the lamellar crystals and relies on two parameters, i.e. the critical value of the free energy needed for nucleation and activation of a screw dislocation in crystallographic planes parallel to the chain axes, and the shear modulus relative to the planes of slip for the dislocations, whereas the role of the interlamellar amorphous phase is neglected. The aim of this study is to analyze to which extend the thickness of the lamellar crystals influences the yield stress for a series of propylene-based copolymers having a well-defined chain microstructure and crystallized in the α and/or γ forms, but possessing different degree of crystallinity, thickness of the lamellar crystals, stability of the crystals and also intrinsic flexibility of the portions of chains belonging to the amorphous regions, in relation with the degree of inclusion (exclusion) of the co-monomers in (from) the crystals. It is shown that, in the case of copolymers with a comonomer concentration lower than a threshold, the yield stress increases with increasing the lamellar thickness regardless of type of comonomer, the relative amount of the two polymorphs (α and γ forms), and the degree of inclusion, in good agreement with the predictions of the CMM approach. For copolymers with comonomer concentration higher than a threshold and bulky side groups, which are excluded from the crystals, the thickness of the lamellar crystals becomes low and the role of the chains in the amorphous regions becomes not negligible. In the case of copolymers with high butene concentration, instead, the stress at yield decreases with increasing the thickness of the lamellar crystals, because the high degree of inclusion of butene units in the crystals induces not only an increase in the thickness of the lamellar crystals, but also a decrease in the stability of the crystals, and the role of the chains located in the interlamellar amorphous layers acting as tie-molecules may not be disregarded.

The development of a highly oriented fiber morphology by stretching low stereoregular isotactic polypropylene (iPP) samples is studied at two different temperatures. The structural and morphological transformations occurring during deformation are followed in real time by collecting in situ wide (WAXS) and small-angle (SAXS) X-ray scattering data. WAXS analysis reveals that the disordered γ form initially present in the samples gradually transforms by stretching into the oriented mesomorphic form of iPP at room temperature and in well-oriented crystals of α form at 60 °C. SAXS analysis indicates that regardless of the stretching temperature, a cascade of events occurs at mesoscale during deformation, consisting in interlamellar separation and lamellar reorientation at low deformations, followed by formation of chevron-like textures, lamellar fragmentation, and cavitation. Moreover, different fibrillary morphologies develop at the different temperatures. In fact, the fibrillary morphology that develops at room temperature is characterized by rod-like fibrillary entities containing mesomorphic aggregates separated by amorphous regions, placed at uncorrelated longitudinal and lateral distances. Instead, the morphology that develops at 60 °C consists of a fibrillary network for the more stereoregular sample and of rod-like fibrillary entities for the less stereoregular sample, and both morphologies include well-oriented crystals of α form alternating with amorphous regions which are periodically stacked along the stretching direction and are placed at uncorrelated lateral distance. We argue that the difference in structural arrangement of amorphous and crystalline regions at mesoscale in the stretched samples are mainly due to the increase of the mobility of the chains in the amorphous matrix at higher temperature, which facilitates the relaxation of the amorphous phase and the changes in the relative arrangement of the crystalline domains.

Innovation of the Ziegler–Natta polymerization process for polymerization of propene that allows tailoring of the mechanical properties of isotactic polypropylene (iPP) directly in the polymerization stage is presented. The catalyst is modified by prepolymerization of trimethylallylsilane or vinylcyclohexane so that the catalyst particles are coated by a thin skin of poly(trimethylallylsilane) or poly(vinylcyclohexane) that will act as a nucleating agent for the crystallization of iPP. The presence of the nucleating agent accelerates the crystallization of iPP and affords crystallization of the α form even upon fast crystallization by quenching the melt, a condition that generally produces crystallization of the mesomorphic form. Crystals of the α form so obtained show a nodular morphology and the absence of the spherulitic superstructure. This novel iPP material is characterized by outstanding and unexpected properties of high mechanical strength and modulus and contemporarily high ductility, flexibility and good transparency due to the nodular morphology of the α form.

Binary blends of samples of syndiotactic polypropylene of different stereoregularity have been prepared by mixing a more stereoregular and crystalline sample sPP1 having [ rrrr ] = 78%, with low stereoregular and poorly crystalline samples sam PP1 and sam PP2 having [ rrrr ] = 54.6 and 45.8%, respectively, and a nearly atactic fully amorphous sample am PP1 with [ rrrr ] = 26.5%. The blends show separate melting of the two components with the more stereoregular component sPP1 melting at high temperature and the poorly syndiotactic component melting at lower temperature. Crystals of sPP1 are formed by cooling from the melt whereas more defective crystals of sam PP1 and sam PP2 are formed by cold crystallization during aging at room temperature. The presence of the strereoirregular component disturbs the melt crystallization of the stereoregular component sPP1 and depression of the crystallization and melting temperatures of sPP1 in the blends is observed. On the other hand, the cold crystallization at room temperature of the irregular component is accelerated and favored by the presence of the more stereoregular component sPP1, whose crystals act as crystallization nuclei. All data indicate interactions between the components in the melt and amorphous state, compatible with a good degree of mixing.

Originally developed on heterogeneous Ziegler-Natta (ZN) catalysts, the model of "chiral growing chain conformation" developed by Corradini to explain the origin of stereocontrol in propene polymerization was extended to all stereoselective polymerization catalysts. The idea that the chiral recognition is performed by the site-chirality thorough the conformation of growing chain represent the "unusual novelty" of ZN towards e.g. asymmetric catalysis in which a chiral host molecule recognize directly two enantiomeric guest molecules. In this paper, by using DFT calculations, we will show that the origin of the stereocontrol for the new generation of Hf(IV)-pyridylamido based catalysts is somewhat different and more similar to the asymmetric catalysis.

A method for assessing the puzzling molecular architecture of ethylene/1-octene multi-block copolymers, synthesized via chain-shuttling technology, is proposed. They are commercial elastomers characterized by alternating segments with low (hard blocks) and high (soft blocks) octene concentration. Our molecular assessment allows determining plausible value for the average length of blocks and number of blocks/chain, setting the basis for a definitive elucidation of structure-properties relationships of these elusive systems. Namely, the molecular basis which explains the similar crystallization properties, and the large difference in mechanical properties of this class of copolymers is unveiled thanks to our approach. We demonstrate that at 25 °C we have similar values of Young modulus and deformation at break higher than 1400%, but large strain hardening before breaking or flat mechanical behavior up to the break, depending on the short or long average block length of the multiblock copolymers, respectively, coupled with a high or low number of blocks/chain, respectively. At 60 °C these differences are amplified entailing no strain hardening but preservation of high ductility for copolymers with short average block length and high number of blocks/chain, remarkable decrease of strain at break of about two orders of magnitude (from ≈2000 to ≈90%) for copolymers with long average block length and lower number of blocks/chain.

The synthesis and the crystal structure of isotactic poly((R,S)-3-methyl-1-pentene) (iP(R,S)3MP) are presented. The synthetic strategy based on hydrogenation of isotactic 1,2-poly(E-3-methyl-1,3-pentadiene) has allowed for the preparation of a purely random copolymer of the two enantiomeric (R)-and (S)-3-methyl-1-pentene monomers. X-ray diffraction analysis and conformational and packing energy calculations have shown that chains of iP(R,S)3MP are in 4/1 helical conformation and are packed in a monoclinic unit cell with parameters a = 10.02 Å, b = 18.48 Å, c = 6.87 Å, and γ = 109.9° according to the space group P2 1 /b or P2 1. A high degree of disorder is present in the crystals due to the random enchainment of the enantiomeric R and S monomeric units, whose chirality influences the handedness of the helical chains and the conformation assumed by the lateral groups. Disorder in the conformation of the lateral groups is also present. The crystal structure of the random copolymer iP(R,S)3MP is different from the tetragonal structure of the pure enantiomer homopolymer iP(S)3MP. This is due to different entropic effects, related to the presence of different types of disorder included in the crystals that drive crystallization in different packing modes. The crystal structure of iP(R,S)3MP provides an example of the effect of the chirality of side groups on the crystal packing and is an example of symmetry breaking in the structures of polymers.

A method is developed for rapid preparation of titania substrates using a sol–gel preparation route in combination with an amphiphilic block copolymer (BCP) template, acting as structure directing agent. The method consists in performing the spin coating of an initial solutions of BCP and titania species in a controlled atmosphere using a flux of solvent vapors (Rapid-Flux-Solvent-Atmosphere, RFSA method). The method is demonstrated in the case of thin films of polystyrene-b-poly(oxyethylene) BCP (PS-PEO) with cylindrical morphology of PEO domains in the PS matrix, characterized by the selective inclusion of titanium species inside the PEO cylinders. Upon removal of the organic matrix by heat treatment at 600 °C, titania substrates of different morphology are obtained depending on the concentration of titania precursor and processing conditions. The method allows preventing the negative effect of hydrolysis and condensation reactions of Ti species and titania substrates with morphology reminiscent of that one achieved in the hybrid nanostructured thin films are obtained. The RFSA method is simple, direct, and it is of particular interest for systems that require a short processing time to eliminate the solvent and to obtain regular nanostructured thin-films over large area. Merit figures such as the degree of hexagonal order and the degree of coverage of the surface with titania motifs are identified to characterize the hybrid composites and titania substrates.

A study of the nucleating effects of isotactic poly(trimethylallylsilane) (iPTMAS) and poly(vinylcyclohexane) (iPVCH) for isotactic polypropylene (iPP) and their influence on the mechanical behavior of iPP is reported. The particles of catalyst are covered by poly(trimethylallylsilane) or poly(vinylcyclohexane) by polymerization of the corresponding monomers before polymerization of propylene. The method guarantees that the nucleating agents are perfectly dispersed into the iPP samples, with their consequent high efficiency. Isotactic poly(vinylcyclohexane) and poly(trimethylallylsilane) favor crystallization from the melt of α and β forms, respectively. Both nucleating agents affect the morphology greatly reducing the size of shperulites. This in turn affects the mechanical properties improving ductility and flexibility. All nucleated samples crystallize by rapid cooling from the melt in the α form and not in the mesophase, as occurs in the pure iPP. The resulting α form presents nodular crystals and behaves as a stiff material with high deformability.

A study of the mechanical properties and of the crystallization behaviour of samples of isotactic polybutene (iPB) of different stereoregularity prepared with metallocene catalysts, is presented. Stereoregular samples with concentration of rr defetcs lower than 2 mol% crystallize from the melt by compression-molding and fast cooling to room temperature in form II, whereas more stereoirregular samples crystallize surprisingly mainly in form III. The stereodefective iPB samples show interesting mechanical properties of high ductility and flexibility that increase with increasing concentration of rr stereodefects, with values of deformation at break higher than 1000% for the most stereoirregular sample containing about 5 mol% of stereodefects. The aging at room temperature produces transformation of form II into form I in more stereoregular samples, whereas no phase transformation occurs in more stereoirregular samples crystallized mainly in form III. A strong increase of Young’s modulus and of stress at yield is observed after transformation of form II into form I. For more stereoirregular samples with concentration of rr defects higher than 2 mol% crystallized in form III, the mechanical properties do not change significantly upon aging at room temperature because no phase transformations occur during aging. Also for the aged samples a remarkable increase of ductility and flexibility with increasing concentration of rr stereodefects is observed. The mechanical deformation is associated with stress induced transformations of form II and form III into form I. These results indicate that mechanical properties of iPB can be modified by the controlled incorporation of stereodefects and improvement of ductility is easily obtained while maintaining high crystallinity by inclusion of small amounts of rr stereodefects.

A set of syndiotactic polypropylene samples with tailored degree of stereoregularity and uniform distribution of stereodefects, obtained via organometallic catalysis, was used to study the relation between the syndiotactic pentad concentration and the viscoelastic plateau modulus in the melt state. The plateau modulus was found to increase with increasing the degree of stereoregularity, indicating that syndiotactic polypropylenes of different stereoregularity produce a different entanglement density of the amorphous phase. The change in plateau modulus was highly nonlinear for large values of syndiotactic pentad contents, suggesting that the presence of even a relatively small number of defects along the syndiotactic backbone significantly alters the space filling attitude of the polymer chain in the melt. This result, which confirms and extends those already obtained from experiments and numerical simulations, points out to the concept that the dynamics of macromolecular chains is largely controlled by the relative configuration of consecutive stereoisomeric centers along the chain, thus providing a link between chain dynamics and molecular architecture

A study of the morphology and the mechanical properties of the mesomorphic form of isotactic polypropylene (iPP) that crystallizes in samples of different stereoregularity prepared with metallocene catalysts is reported. Highly isotactic samples slowly crystallized from the melt in the α form show the typical lamellar morphology with organization in spherulites. Bundle-like elongated crystalline entities and needle-like crystals of γ form are instead observed for stereoirregular samples slowly crystallized from the melt in the γ form. All samples crystallize by fast quenching the melt at 0 °C in the mesomorphic form, regardless of stereoregularity. Crystals of the mesomorphic form always exhibit a nodular morphology and absence of lamellar spherulitic superstructures, independent of the stereoregularity. This morphology accounts for the similar good deformability of all the quenched samples, whatever the concentration of stereodefects. For all samples and any stereoregularity, the nodular morphology with absence of spherulites is preserved after annealing and transformation of the mesophase into α form. The formation of a nodular α form accounts for outstanding properties of high ductility and mechanical strength of the annealed samples crystallized in the α form. The most stereoirregular sample with rr concentration of 11% shows elastic behavior either when it is slowly crystallized in the γ form or when it is crystallized in the mesomorphic form and also after annealing and transformation of the mesophase into the nodular α form.

Polyethylene-block-syndiotactic polypropylene (PE-block-sPP) crystalline–crystalline block copolymers with different block lengths have been synthesized with a stereospecific living organometallic catalyst. Samples of PE-block-sPP have been epitaxially crystallized onto crystals of p-terphenyl (3Ph) to achieve a control over the crystallization of both blocks and study the dependence of the thin film morphology on the sequential crystallization of the two blocks by cooling from the melt. The epitaxial crystallization generates oriented overgrowth of both crystals of sPP and PE, with a highly ordered single orientation of sPP lamellae and a double orientation of PE lamellae onto the (001) face of 3Ph. The final morphology depends on which polymer block crystallizes first, a sequence that depends on the block copolymer composition and block lengths. Ordered nanostructures with alternating lamellar domains are obtained and oriented by the lamellae that crystallize first.

Summary Block-copolymers (BCPs) self-assembling provides a unique tool for realizing large-area ordered metamaterials with desired optical properties due to the rich range of tunable nano-morphologies and to the chemical properties of the block polymers that can lead to the selective inclusion of functionalized nanoparticles (NPs) in specific nanodomains, generating periodic arrays of NPs according to the geometry of the BCP acting as template. In this work, we investigate nanostructured thin films of polystyrene-block-poly(methylmethacrylate) (PS-PMMA) BCP filled with functionalized metallic (Au) nanoparticles. The BCP is characterized by an hexagonal array of PS cylinders in the PMMA matrix and the Au NPs are selectively included in the PS cylindrical domains. The optical properties of such nano-structures are strongly affected by localized surface plasmons (LSPs) in the NPs. We theoretically demonstrate that the optical properties of such nano-structures can be tuned by varying the NPs loading concentration and the NPs radius. Then, we investigate the effects of the hexagonal geometry on the collective plasmonic oscillations. We fabricated the PS-PMMA/Au NPs composite and we experimentally demonstrate the existence of collective plasmonic resonances of gold nanoparticles after an electromagnetic stimulation, arising from the ordered arrangement of AuNPs in the periodic array of hexagonally packed cylinders created by BCP self-assembly. This approach allows a fine modulation of the NPs optical properties and can be used as an intriguing and versatile tool to build useful devices for Optics & Photonics applications, with significant benefits for both fundamental and applied investigations.

A study of the structure and properties of binary blends of syndiotactic polypropylene (sPP) having different degrees of stereoregularity and synthesized by different metallocene catalysts, is reported. The blends have been prepared by mixing a stereoregular and crystalline sample with [ rrrr ] = 78% with low stereoregular and poorly crystalline samples having [ rrrr ] = 54.6 and 45.8%, and a nearly atactic fully amorphous sample with [ rrrr ] = 26.5%. All blends show outstanding elastomeric properties with rigidity and tensile strength that can be tailored by changing the stereoregularity of the components and blend composition. A continuous change of properties with composition is observed, from those of high-modulus thermoplastic elastomers for blends having high concentration of the most syndiotactic sample sPP1, to those of low-strength elastomers for blends with high content of the less syndiotactic component. The addition of the stereoirregular component to the more stereoregular sample produces an effect on the crystallization behavior and mechanical properties analogous to that produced by incorporation of defects of stereoregularity. Blends behave as a single sPP component having an average degree of stereoregularity intermediate between those of the two components. All data indicates a good miscibility of the sPP chains of different stereoregularity in the melt and in the amorphous state.

When in contact with water, poly(lactic acid), PLA, undergoes severe physical changes. Hydrolysis is obviously the most expected and studied one, and it is followed by weight loss. However, other phenomena occur such as opacification and crystallization. Although these phenomena are sometimes reported in the literature, apart from hydrolysis which has been thoroughly studied at several temperatures, the other ones have not been deeply analyzed and their timescales have not been so far determined. This work aims at studying the physical changes induced by water sorption to PLA, by analyzing the modifications taking place in solid samples immersed in water for different times (starting from a few hours) and at temperatures close to the glass transition. The obtained results allow determining the timescales of the different phenomena and thus the possible cause-effect relationship among them. In particular, it is shown that opacity is the first phenomenon to become evident, on a timescale consistent with water sorption, and seems to be due to crazes induced by water. The second phenomenon to become evident is the crystallinity evolution, which is shown to be consistent with a heterogeneous nucleation on the crazes. These two phenomena take place before hydrolysis becomes significant, thus raising doubts about the fact that hydrolysis can be their cause.

The crystallization behavior and the mechanical properties of random isotactic butene–octene copolymers (iPBC8) prepared with a metallocene catalyst are presented. Samples with low octene concentrations, up to nearly 6 mol%, crystallize from the melt in form II. In these samples octene units stabilize the tetragonal form II, decelerating or even preventing the transformation of form II into the trigonal form I. Octene concentrations higher than 7 mol% prevent crystallization of the samples from the melt and the obtained amorphous samples crystallize by aging at room temperature in mixtures of form II and form I'. The sample with the highest octene concentration of 11.5 mol% crystallizes surprisingly from the amorphous phase directly in the pure form I'. In these samples, form I' crystallized from the amorphous phase transforms into form II by stretching. Form II transforms back into form I' upon releasing the tension. Therefore, in samples where form I' is favored in powder samples, form II is favored in stretched fibers. The transformation of form I into form II by stretching and the back transformation of form II into form I after releasing the tension is unusual for isotactic polybutene (iPB) and rather unexpected. The presence of octene comonomer units also affects the mechanical behavior of iPB and produces increase of flexibility and ductility with increasing comonomer content, and samples with octene concentration higher than 6–7 mol% show unexpected remarkable elastomeric properties. The formation of crystals of form II or form I' during aging allows development of elastomeric properties and enhancement of mechanical strength. The elastic properties are associated to the reversible crystal–crystal phase transition between form II and form I occurring during stretching and when the tension is removed.

The structure and thermal properties of a new class of isotactic copolymers of ethylene (E)/4-methyl-1,3-pentadiene (4MPD) with 4MPD content in the range 40–73 mol % and of isotactic poly-1,2-(4-methyl-1,3-pentadiene) (iP4MPD) homopolymer have been analyzed. The copolymers have been synthesized using 1,4-dithiabutanediyl-linked bis(phenolate) titanium complexes as catalyst precursors, activated by methylaluminoxane. Solution 13C NMR analysis reveals the tendency of the catalyst system to produce copolymers with alternating 4MPD/E constitution. All copolymers are crystalline and show melting temperatures in the range 90–100 °C. The samples with nearly equimolar concentration of units crystallize in a new crystalline form because of the alternating constitution. Structural analysis indicates that in this new form the alternating 4MPD-E sequences assume a nearly trans-planar conformation with periodicity c = 5.13 Å and are arranged in a monoclinic unit cell with parameters a = 5.70 Å, b (unique axis) = 14.95 Å, c (chain axis) = 5.13 Å, and β = 114.4°, according to the space group symmetry Pn.

The crystal structure of the alternating copolymer of limonene epoxide and carbon dioxide has been studied. Highly stereoregular and regioregular alternating copolymers have been prepared starting from cis/trans mixtures of both enantiopure (R) and (S) isomers of 1-methyl-4-(1-methylvinyl)-7-oxabicyclo[4.1.0] heptane (limonene oxide) using a β-diiminate zinc complex under mild conditions of temperature and pressure, to yield enantiopure poly(1S,2S,4R-limonene carbonate) (PRLC) and poly(1R,2R,4S-limonene carbonate) (PSLC) products, respectively. Attempts to crystallize the pure enatiomers failed, whereas racemic mixtures readily crystallize from solution, forming racemic crystals. In the crystals, enantiomorphic chains are packed in an orthorhombic unit cell with axes a = 9.71 ± 0.05 Å, b = 10.68 ± 0.05 Å, and c = 11.31 ± 0.05 Å (chain axis) according to the space group Pbc21 and 2 chains (4 monomeric units)/unit cell. The chain periodicity c of 11.31 Å can be accounted for by a s(2/1) helical conformation with nearly all-trans bonds in the backbone. Isoclined chains of opposite chirality are packed in the unit cell forming well interdigitated bc layers piled along a via zipper interactions of the enantiomorphic side groups belonging to adjacent chains. The structure is characterized by the presence of different types and degrees of disorder that arises from the nearly random rotation of isopropenyl groups around the connection bond to the cyclohexane rings (conformational disorder), twisting of cyclohexane cycles, and up/down positional disorder of isochiral chains in the lattice positions (substitution type disorder). It is argued that the formation of racemic crystals instead of a mixture of enantiopure crystals is stabilized by the favorable interactions of the polar carboxyl groups and the zipper interactions of the lateral chiral groups of first neighboring chains along b. It is also inferred that the crystallization of enantiopure crystals is prevented by the slow crystallization kinetics and the less favorable interactions between chains in an isochiral packing.

A study of the crystallization behavior and mechanical properties of random isotactic butene–ethylene copolymers prepared with a metallocene catalyst is presented. The use of the metallocene catalysis ensures a fine control over the molecular structure with low concentration of rr stereodefects (0.8%), negligible amount of regiodefects, and random and uniform distribution of ethylene constitutional defects. This molecular characteristic has allowed evidencing the only effect of the presence of ethylene units on the polymorphic behavior and mechanical properties of isotactic polybutene (iPB). The presence of ethylene accelerates the transition of form II into form I at room temperature and at concentration of nearly 6 mol % favors the direct crystallization from the melt of the stable form I. The presence of ethylene also affects the mechanical behavior of iPB and produces increase of flexibility and ductility with increasing ethylene content. A significant modification of the properties of iPB is observed...

Graphical Abstract Aqueous poly(vinyl alcohol) (PVA) solutions subjected to cryogenic treatment form strong physical gels. The cryogenic treatment basically consists of freezing an initially homogeneous polymer solution at low temperatures, storing in the frozen state for a definite time, and defrosting. These gels are of great interest for biotechnology, medicine, the food industry, and many other applications. The outstanding properties of these systems depend on a complex macroporous architecture, whereby PVA chains and water molecules are organized over different hierarchical length scales. The structure and the principal processes subtending the formation of these systems are discussed in the framework of our current understanding of polymer gels. These processes involve formation of ice crystals, PVA crystallization, liquid–liquid phase separation, hydrogen bonding, and entanglements. Small angle neutron scattering is used to follow the cryotropic gelation of PVA/water solutions and detailed information is extracted concerning the gelation mechanism and kinetic parameters related to the formation of these complex systems. Open image in new window

A strategy to tailor the properties of isotactic polypropylene, based on the combination of the crystallization of the mesophase and the control of the molecular structure and concentration of stereodefects via polymerization with metallocene catalysts, is presented. The mechanical properties can be modulated through the conventional slow crystallization from the melt in the α and γ forms, depending on the concentration of stereodefects, or the crystallization of the mesophase by melt quenching. The most stereoirregular sample with rr concentration of 11% shows elastic behavior either when it is slowly crystallized in the γ form or when it is crystallized in the meso morphic form, but with remarkable differences in the values of the modulus and tensile strength.

A study of the thermodynamic stability and the related polymorphic transformations induced by thermal treatments of the mesomorphic form that crystallizes in stereodefective metallocene isotactic polypropylene (iPP) is presented. We show that the mesomorphic form of the more isotactic samples is stable at room temperature, whereas the mesomorphic form crystallizing in the more stereoirregular sample is unstable and crystallizes at room temperature in the crystalline α form. In any case, the mesomorphic form transforms during heating or by annealing at temperatures higher than 60–80 °C always in the α form, regardless of the stereoregularity, even in the case of stereoirregular samples generally crystallizing from the melt in the γ form. These data confirm the proposed model of structure of the mesomorphic form as small aggregates of chains in three-fold helical conformation packed with lateral correlations similar to the α form of iPP.

The crystal structure of syndiotactic polystyrene (sPS) co-crystals of δ -class is discussed with the aim of identifying molecular descriptors that allow predicting the lattice distortions induced by the inclusion of guest molecules inside the cavities of the nanoporous δ -form. Based on the consideration that monoclinic and triclinic clathrates and intercalates of sPS form as a result of the unique deformability of the cavities of the δ -form, the identifi ed parameters are the fractional occupancy factors of the undeformed cavity of the δ -form from guests along the a -axis, the c -axis, and the normal to the (010) planes of the crystals, and the relative orientation of the guests to the chain axes.

Random copolymers of syndiotactic polypropylene (sPP) with hexene and octene comonomeric units have been synthesized in a wide range of comonomer concentrations with a single-center syndiospecific Cs-symmetric metallocene catalyst. The effects of the presence of hexene and octene on the crystallization behavior of sPP and in particular on the stress-induced transformations have been analyzed. All copolymer samples crystallize from the melt in the stable helical form I of sPP. At very low concentrations of hexene or octene (around 1-2 mol %) the behavior of these copolymers is similar to that of sPP; that is, the stable helical form I of the melt-crystallized samples transforms into the trans-planar form III by stretching at high deformations. For hexene or octene concentrations in the range 2-5 mol % the stretching produces transformation of the helical form I into the trans-planar mesophase. Finally, for hexene or octene concentrations higher than 4-5 mol%the helical form I transforms by stretching at high deformation into a different mesomorphic form, characterized by chains in the ordered 2/1 helical conformation, and disorder in the lateral packing of the chains. This is a first evidence of the crystallization of a solid mesophase of sPP with chains in 2-fold helical conformation, never observed before in the sPP homopolymer.

The structure and properties of blends of poly(ethylene terephthalate) (PET) with poly(trimethylene terephthalate) (PTT) at PTT concentration ≤30 wt.%, obtained with three different methods: from solution, melt extrusion, and direct spinning, are investigated. Relationships between the method of preparation and properties of blends are established. All blends show glass transition temperature at values determined by composition, and crystallization properties also dependent on the preparation method. Blends obtained from solution show separated melting of components. For blends obtained from the melt only PET crystallizes. The melting temperature decreases with the residence time of the melt at high temperatures, due to occurrence of ester exchange reactions. It is shown that reactive blending of PET/PTT mixtures occurring during preparation is a versatile route for obtainment of engineering materials with good mechanical properties, high crystallinity, glass transition temperature lower than that of PET, and melting temperature that may be controlled by the processing conditions.

Isotactic propylene−1-octene (iPPC8) and propylene−1-octadecene (iPPC18) copolymers have been synthesized with a highly isospecific C2-symmetric metallocene catalyst. iPPC8 copolymers crystallize in the α form of iPP for octene concentration up to 12−13 mol %, whereas iPPC18 copolymers crystallize in the α form for very low octadecene concentration and in a new mesomorphic form for octadecene contents higher than 2−3 mol %. In both copolymers crystals of α form transform into the new mesophase by stretching at high deformations. This mesophase is different from the quenched mesomorphic form of the iPP homopolymer. In fact, the diffraction patterns of the new mesophase of iPPC18 copolymers present a strong equatorial reflection at 2θ = 20°, which is absent in the diffraction pattern of the mesophase of iPP. The new mesophase is characterized by parallel chains in 3/1 helical conformation packed at average interchain distances of about 6 Å, defined by the self-organization of the flexible side groups, and high degree of disorder in the lateral packing of the chains

A study of the crystallization from the melt and of the polymorphic behavior of isotactic poly(1-butene) (iPB) prepared with metallocene catalysts is presented. Samples of isotactic polybutene of different stereoregularity, containing different concentrations of stereodefects (rr triads defects) and regiodefects (4,1 units) have been prepared with different C2- and C1-symmetric metallocene catalysts. The concentration of defects and the molecular mass are controlled through a rational choice of the catalyst structure and conditions of polymerization. Highly isotactic samples with concentration of rr stereodefects lower than 2 mol %, crystallize from the melt in the metastable form II. This is a common polymorphic behavior observed in iPB samples prepared with Ziegler-Natta catalysts and extensively reported in the literature. More stereodefective iPB samples, containing concentration of stereodefects higher than 2 mol %, crystallize form the melt as mixtures of form II and form I, the fraction of crystals of form I increases with increasing concentration of defects and decreasing cooling rate from the melt, and samples with concentration of rr defects of 3-4 mol%crystallize from the melt in the pure form I at low cooling rates. This is the first experimental observation of the crystallization of the stable trigonal form I of iPB from the melt at atmospheric pressure. This result may be of interest because the crystallization of the metastable form II from the melt and the spontaneous transformation at room temperature of form II into the stable form I has been considered for long time an unavoidable problem that stands in the way of the industrial development of the Ziegler-Natta iPB.

We report a new class of catalysts for the enantioselective copolymerization of CO2 and alicyclic meso epoxides. C1-Symmetric β-diiminate zinc catalysts were designed on the basis of mechanistic information and subsequently optimized using structure–activity relationships and iterative ligand design. The optimized catalysts yield highly isotactic poly(cyclohexene carbonate) and poly(cyclopentene carbonate) with units of up to 94% ee under mild conditions. A diblock copolymer of CO2/cyclohexene oxide and CO2/cyclopentene oxide was synthesized, demonstrating the controlled behaviour of the catalyst. Solid-state structures and solution-state dynamics of the catalyst were studied to explain trends in enantioselectivities and turnover frequencies. A strong correlation between the isotacticity of the polymer and its melting temperature was observed. X-ray powder diffraction was used to measure crystallinity and study the changes in morphology observed upon annealing.