Metallocene ethylene-1-octene copolymers: Effect of extrusion conditions on thermal oxidation of polymers with different comonomer content

Metallocene ethylene-1-octene copolymers having different densities and comonomer content ranging from 11 to 36 wt% (m-LLDPE), and a Ziegler copolymer (z-LLDPE) containing the same level of short-chain branching (SCB) corresponding to one of the m-LLDPE polymers, were subjected to extrusion. The effects of temperature (210-285 °C) and multi-pass extrusions (up to five passes) on the rheological and structural characteristics of these polymers were investigated using melt index and capillary rheometry, along with spectroscopic characterisation of the evolution of various products by FTIR, ¹³C-NMR and colour measurements. The aim is to develop a better understanding of the effects of processing variables on the structure and thermal degradation of these polymers. Results from rheology show that both extrusion temperature and the amount of comonomer have a significant influence on the polymer melt thermo-oxidative behaviour. At low to intermediate processing temperatures, all m-LLDPE polymers exhibited similar behaviour with crosslinking reactions dominating their thermal oxidation. By contrast, at higher processing temperatures, the behaviour of the metallocene polymers changed depending on the level of comonomer content: higher SCB gave rise to predominantly chain scission reactions whereas polymers with lower level of SCB continued to be dominated by crosslinking. This temperature dependence was attributed to changes in the different evolution of carbonyl and unsaturated compounds including vinyl, vinylidene and trans-vinylene.     

A literature review on the in situ generation of reinforcing fibers

Liquid crystal polymers (LCPs) are a relatively new class of materials. These polymers usually consist of rigid rodlike molecular chains and they are capable of forming highly oriented structures even in the as-made product, with strength/modulus significantly higher than those of the conventional flexible chain polymers. Blending of LCPs with conventional polymers produces composite-like structures with LCPs serving as the reinforcing component. The properties of the blends are affected by the size, shape and distribution of the LCPs in the matrix polymer, which in turn are related to the processing conditions such as the blend composition, the extrusion and drawing conditions, the viscosity ratio of the component polymers and the type and grade of the LCPs and the matrix polymers. Improved processability of the blend due to the reduction in viscosity and the improved interfacial adhesion between reinforcing fibers and the matrix polymer are among the advantages of these materials over the conventional short fiber reinforced composites. This paper gives a brief review of the work currently available in the literature on rheology, fabrication, blend morphology and mechanical/thermal properties of the in situ composites from blends of LCPs and conventional polymers.     

Thermal conductivity behaviour of polymers around glass transition and crystalline melting temperatures

The thermal conductivity of five semi-crystalline and four amorphous polymers was determined within a wide range of temperature, starting at room temperature and going up to temperatures above the polymer melting point (Tm) for semi-crystalline polymers or above the glass transition temperature (Tg) for amorphous polymers. Two transient techniques were employed in the experimental investigation: the hot wire technique for the group of amorphous polymers, and the laser flash technique for the semicrystalline polymers. As expected, the experimental results show that Tg exerts a measureable influence on the thermal conductivity of amorphous polymers. In the case of semi-crystalline polymers, a singular behaviour of the thermal conductivity is observed within the Tm range. In order to explain the anomalous behaviour, the influence of these transition temperatures on the thermal conductivity behaviour with temperature has been analysed in terms of a phonon conduction process and temperature variations of specific heat and modulus of elasticity of the analyzed polymers.     

Unified study of the different physical properties of amorphous polymers

Uptil now it has not been possible to explain the different physical properties of amorphous polymers using a model based on a single conceptual scheme. In this paper, a phenomenological model is proposed which tries to explain the mechanical, optical and thermal properties (both thermal conductivity and expansivity) of amorphous polymers. The model has similarities with the composite model, proposed by the present authors, which has proved to be successful in interpreting the different physical properties of semicrystalline polymers. The present model considers the bulk form of the polymer as an aggregate of microscopic units possessing intrinsic physical properties. On drawing, the development of anisotropy in different physical properties is supposed to be due to the development of preferred orientation of these units. The development of the preferred orientation has been estimated directly from birefringence data. The agreement between the calculated and experimental values of the elastic modulus, thermal conductivity and thermal expansivity of PVC, PMMA and PS is found to be reasonable good.     

Morphology and tensile property relations of high-strength/high-modulus polyethylene fiber

High-strength/high-modulus polyethylene (PE) fibers have been obtained from linear polymers by melt-spinning the polyethylene followed by hot-drawing the spun filaments. Fibers with different tensile properties were made by changing the extrusion, solidification, and drawing conditions and by use of different types of polyethylene. Potential correlations of fiber strength with morphological parameters such as crystallite dimensions, and crystalline and amorphous orientation were examined. The integrated meridional small-angle x-ray scattering intensity is the only morphological parameter which was found to correlate well with the strength of fibers prepared under different conditions and from different polymer types. Implications with respect to the molecular mechanism of PE fiber drawing are also discussed.     

Miscibility Behaviour of Poly(ether Sulphone)/nylon-6 Blends

Binary blends of poly (ether sulphone) (PES) and Nylon-6 were prepared in a whole range of composition by melt extrusion. Miscibility behaviour of the blends were studied using thermal analytical techniques like differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). Due to the rapid crystallization of Nylon-6 as it is cooled from the melt state, its glass transition behaviour could not be detected even in the quenched samples by DSC. Furthermore, the crystallization and melting behaviour of the blends have been studied by DSC. DMA results show that the dynamic storage modulus of the blends were in-between those of the constituent polymers. Also the glass transition of Nylon-6 phase as determined by the peak in loss tangent remains constant which shows that the two polymers are immiscible. Thermal expansion coefficient of the blends as determined by TMA is greater than that of Nylon-6 signifying the increased dimensional stability of the blends at higher temperatures. Morphological studies done by scanning electron microscopy (SEM) show the biphasic nature of the blends, with clear cut boundaries between the phases because of poor interfacial adhesion. Dispersed particle size is small when Nylon-6 is the dispersed phase because of its lower melt viscosity as compared to PES. Thermal stability of the blends was measured using thermogravimetric analysis (TG). Two-step decomposition behaviour was observed because of macro-phase separated morphology. This revised version was published online in July 2006 with corrections to the Cover Date.     

Comportement thermique de polyamides et polysulfonamides “semi-aromatiques”

Various techniques (thermal analysis, dilatometry, X-ray diffraction, nuclear magnetic resonance, mass spectrometry, electrical measurements) have been used to study the thermal behaviour of several “half-aromatic” polyamides and polysulphonamides. Three transition phenomena were revealed and related to morphological modification of polymers.     

Raman spectroscopy for polymer characterization in an industrial environment

In this paper we discuss the use of Raman spectroscopy for characterising polymers both in the laboratory and also in-situ at the production line. We show how polymer crystallinity can be followed during extrusion and drawing, and describe the compositional analysis of cross-linked acrylic terpolymers in a polymerisation reactor. We also discuss problems which can arise such as sample fluorescence from moving polymers, distortion of relative band intensities due to chromatic aberration, and sampling difficulties with turbid solutions.     

Liquid-crystalline main chain polyesters derived from p-phenylenediacrylic acid

4,4′-(p-phenylenediacryloyloxy)-dibenzoyl chloride was polycondensed at 220°C with several diols in order to obtain liquid-crystalline polymers. The mesomorphic properties were studied by optical microscopy and differential scanning calorimetry. Gelation due to thermal crosslinking occurred at higher temperatures. With the exception of 2,2′-biphenyldiol derivative, all the polymers derived from aliphatic diols, dihydroxy ethers and aromatic diphenols exhibit mesogenic behaviour.     

Novel photo-crosslinkable liquid crystalline polymers containing vanillylidene cycloalkanones and azobenzene units

Two series of novel liquid crystalline photo-crosslinkable bis(vanillylidene-azobenzene) cycloalkanone containing polymers, namely poly(vanillylidene alkyloxy-4,4'-azobenzenedicarboxylic ester)s, have been synthesised from bis[m-hydroxyalkyloxy(vanillylidene)cycloalkanone] (m = 6, 8, 10) with azobenzene dicarbonylchloride by solution polycondensation method at ambient temperature. Polymers with varying spacer lengths have been synthesised and characterised by spectroscopic techniques. These variations have been correlated with the thermal properties and transition temperatures. Thermal transitions were analysed by differential scanning calorimetry (DSC) and the mesophases were identified by hot stage optical polarised microscopy (HOPM). All of the polymers were found to exhibit liquid crystalline properties. Transition temperatures were observed to decrease with increasing spacer length. The thermogravimetric analysis reveals that all of the polymers were stable up to 280°C undergo two-stage decomposition. Using the UV-visible photolysis studies we investigated the simultaneous behaviour of reactivity rates of crosslinking in the vanillylidene unit and isomerisation caused by the azobenzene unit in the photo-crosslinkable main chain liquid crystalline polymers. The photolysis of liquid crystalline bis(vanillylidene)cycloalkanone-based polymers reveals that there are two kinds of photoreactions in these systems: the EZ photoisomerisation of azobenzene unit and 2p+2p addition by vanillylidene units. The EZ photoisomerisation in the liquid crystal phase disrupts the parallel stacking of the mesogens, resulting in the transition from the liquid crystal phase to isotropic phase. The photoreaction involving 2p+2p addition of the bis(vanillylidene)cycloalkanone units in the polymers results in the conjoining of the chains. The cyclopentanone polymers exhibited a faster rate of photolysis than the cyclohexanone polymers.     

Thermal studies of polyvinylcarbazole and polyvinyldibromocarbazole

The thermal stabilities and behaviour of polyvinylcarbazole and polyvinyldibromocarbazole were investigated by a derivatograph. The initial and final temperatures of decomposition of these polymers were determined and their thermal properties compared.     

Synthesis,thermal and optical properties of new nanosized polyamides containing N-phenyl- and O-naphthyl-s-triazine rings; part 2

We report the chemistry and properties of two new series of well-defined nano sized spheres aramides-containing N- and O-naphthyl-s-triazines. The polymers were carefully characterized by different techniques including infrared, ultraviolet, fluorescent emission, elemental, thermal analyses and scanning electron microscopy (SEM). The polymers were readily soluble in polar aprotic solvents while insoluble neither in water nor halogenated solvents. Thermal analyses data up to 900°C showed high thermal behavior and the polymers were classified either as “slow burning polymers” or “self-extinguishing polymers” based on their calculated the limiting oxygen index. Interestingly, the naphthyl / phenyl interchange has dramatic improvement on the thermal properties. Obviously, the pyridine / phenylene interchange has no influence on the thermal properties of the addressed polymers. Thermal stability of the aniline-containing polymers proved to be comparable to their naphthylamine analogues. Polymers containing p-phenylene moieties exhibited better thermal results compared to their analogues containing m-phenylene moieties. Benzidine containing polymers and sulfone containing polymers exhibited better thermal stabilities than their analogues containing either ether or methylene flexible linkages. The kinetic data obtained from the nonisothermal decomposition of the prepared polyamides series were also studied. The polymers exhibited emissions ranging from blue to orange wavelength depending on the nature of the signaling unit. The naphthyl / phenyl interchange led to either appreciable red-shifted absorptions in some cases or blue-shifted absorptions in other cases and this behavior may be attributed to the contorted, twisted structural nature of the naphthalene ring. Such attracting properties make these polymers good candidates for applications such as processable high-temperature materials and also as heat-resistant polymeric materials.     

Metallocene ethylene-1-octene copolymers: Influence of comonomer content on thermo-mechanical, rheological, and thermo-oxidative behaviours before and after melt processing in an internal mixer

The influence of the comonomer content in a series of metallocene-based ethylene-1-octene copolymers (m-LLDPE) on thermo-mechanical, rheological, and thermo-oxidative behaviours during melt processing were examined using a range of characterisation techniques. The amount of branching was calculated from ¹³C NMR and studies using differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) were employed to determine the effect of short chain branching (SCB, comonomer content) on thermal and mechanical characteristics of the polymer. The effect of melt processing at different temperatures on the thermo-oxidative behaviour of the polymers was investigated by examining the changes in rheological properties, using both melt flow and capillary rheometry, and the evolution of oxidation products during processing using infrared spectroscopy.The results show that the comonomer content and catalyst type greatly affect thermal, mechanical and oxidative behaviour of the polymers. For the metallocene polymer series, it was shown from both DSC and DMA that (i) crystallinity and melting temperatures decreased linearly with comonomer content, (ii) the intensity of the β-transition increased, and (iii) the position of the tan δ_max peak corresponding to the α-transition shifted to lower temperatures, with higher comonomer content. In contrast, a corresponding Ziegler polymer containing the same level of SCB as in one of the m-LLDPE polymers, showed different characteristics due to its more heterogeneous nature: higher elongational viscosity, and a double melting peak with broader intensity that occurred at higher temperature (from DSC endotherm) indicating a much broader short chain branch distribution.The thermo-oxidative behaviour of the polymers after melt processing was similarly influenced by the comonomer content. Rheological characteristics and changes in concentrations of carbonyl and the different unsaturated groups, particularly vinyl, vinylidene and trans-vinylene, during processing of m-LLDPE polymers, showed that polymers with lower levels of SCB gave rise to predominantly crosslinking reactions at all processing temperatures. By contrast, chain scission reactions at higher processing temperatures became more favoured in the higher comonomer-containing polymers. Compared to its metallocene analogue, the Ziegler polymer showed a much higher degree of crosslinking at all temperatures because of the high levels of vinyl unsaturation initially present.     

Thermal characterization of HCN polymers by TG-MS, TG, DTA and DSC methods

This paper presents a thermogravimetry (TG) study of hydrogen cyanide polymers, synthesized from the reaction of equimolar aqueous solutions of sodium cyanide and ammonium chloride. Differential thermal analysis (DTA) and differential scanning calorimetry (DSC) were also used to evaluate the thermal behaviour of these black polymers, which play an important role in prebiotic chemistry. A coupled TG-mass spectrometer (MS) system allowed us to analyze the principal volatile thermal decomposition and fragmentation products of the isolated HCN polymers under dynamic conditions and an inert atmosphere. After dehydration, a multi-step decomposition occurred in this particular polymeric system, due to the release of ammonia, hydrogen cyanide (depolymerization reaction), isocyanic acid (or cyanic acid) and formamide; these two latter species allow us identify bond connectivities. Finally, data collected from TG experiments in an oxidative atmosphere showed significant differences at higher temperatures, above 400 °C. According to these results, the different techniques of thermal analysis here applied have demonstrated to be an adequate methodology for the study and characterization of this complex macromolecular system, whose structure remains controversial even today.     

Bioresorbable tyrosol-derived poly(ester-arylate)s with tunable properties

We designed and developed a novel library of tyrosol-derived poly(ester-arylate)s that exhibit tunable chemical, thermal, mechanical, and degradative properties. To build the library, the diphenols 4-hydroxyphenethyl 2-(4-hydroxyphenyl)acetate (HTy) and 4-hydroxyphenethyl 3-(4-hydroxyphenyl)propanoate (DTy) are synthesized and subsequently polymerized with various diacids. Characterization of library members is performed in order to assess the impact of chemical structure on polymer properties. Specifically, the relative influence of diphenol pseudosymmetry versus asymmetry, diacid carbon chain length, and diacid bond rigidity on resulting properties is investigated. Diphenol choice greatly impacts resulting polymer thermal properties and processability. HTy-containing polymers generally have lower melting temperatures compared to their DTy-derived counterparts and are easier to quench in the amorphous phase. As a result, processing results in greater tunability for HTy-derived polymers. One specific example was pHTy3, which increased its tensile modulus from 1 GPa to 3 GPa upon drawing. Diacid lengths and bond rigidity also significantly influence thermal, mechanical, and degradative properties. In all, members of this library can be synthesized efficiently, with high molecular weight and exhibit a wide range of properties, motivating future commercial translation.     

The synthesis and thermal properties of polyepichlorohydrin side-chain liquid crystal polymers

The synthesis and characterization of a new series of side-chain liquid crystal polyepichlorohydrin (PECH) polymers are described. The structures and thermal properties of the synthesized polymers were investigated using IR, NMR, polarized optical microscopy and differential scanning calorimetry. A substantial increase of the glass transition temperature with the degree of substitution of side-chain groups was observed. Polymers with a degree of substitution of side groups, of at least 60%, exhibited thermotropic liquid crystalline behaviour. The polymers present thermal liquid crystalline behaviour and form Schlieren and thread texture upon cooling from the isotropic phase, after annealing for 120 min at different temperatures. In addition, the thermal decomposition of PECHOPhPhCN was studied by thermogravimetry under both nitrogen and air environments. The temperature of the maximum decomposition rate was about 340 °C. Weight loss decreased considerably after 350 °C and was approximately 98% at 700 °C. Chemical modification of functional polymers offers a simple method for obtaining liquid crystalline polymers whose transition temperature can be tailored by varying the amount of substitution, however complete substitution cannot be achieved.     

Thermal behaviour of poly(dimethylsiloxane) hybrid silicas prepared by radiation grafting

This paper reports our investigation regarding the thermal properties of new polymer-silica hybrid materials obtained by radiation grafting. The polymer poly(dimethylsiloxane),bis(3-aminopropyl)terminated is γ-grafted on a silica gel surface. The thermal behaviour of γ-grafted hybrid materials reveals remarkable differences compared to the thermal behaviour of physically adsorbed polymers. These differences allow us to assess the ability of γ-rays to produce a polymer chemically bonded on a silica surface. The chemical bonds formed by irradiation give to the polymer a high conformational stability confirmed by DTA analysis.     

Inorganic-Organic Hybrid Polymers Based on Surface-Modified Metal Oxide Clusters

Transition metal oxide clusters with unsaturated carboxylate ligands bonded to their surface were polymerized in the presence of organic co-monomers by various polymerization techniques to form cluster-reinforced polymers. The properties of the cluster-crosslinked hybrid polymers are distinctly different to those of the parent polymers and originate from both crosslinking and filler effects. Variation of the cluster proportion, the kind of employed cluster, the ratio of functional and non-functional capping ligands and the polymerization conditions allows modifying the materials properties of the hybrid materials. The most important changes in materials properties compared with the cluster-free polymers relate to the swelling behavior, thermal stability and mechanical properties. Furthermore, cluster-specific properties can be introduced into the polymers, such as magnetic properties.     

The use of synchrotron X-ray scattering coupled with in situ mechanical testing for studying deformation and structural change in isotactic polypropylene

The mechanical behaviour of semi-crystalline polymers is greatly influenced by the properties of the crystalline and the amorphous phases. As a result this topic has been the subject of extensive research. However, to date, a comprehensive relationship between the structure and mechanical properties for semi-crystalline polymers has yet to be established. This present study concerns the commissioning of a novel method for in situ data collection during the deformation of polymers. This involves the combination of three different techniques into a single experiment, namely tensile testing, synchrotron radiation wide angle X-ray scattering, and optical microscopy. For this current investigation, three isotactic polypropylene samples have been studied, produced using different thermal treatments. This enables the influence of thermal treatment on the mechanical properties and crystallographic structure to be assessed. The results indicate that tensile properties are influenced by thermal treatment via the relative fraction of -phase material in the sample. As the temperature increases at which thermal treatment takes place, iPP ductility decreases due to the greater rigidity of the increasing -phase content. Differences in crystal strain between the different iPP crystal phases are also observed although the reasons for such differences remain unclear.     

Thermal and flammability properties of polyethersulfone/halloysite nanocomposites prepared by melt compounding

This paper presents a study of polyethersulfone (PES)/halloysite nanotube (HNTs) nanocomposites prepared by melt compounding either through a simple extrusion process or via a water-assisted extrusion procedure. Scanning and transmission electron microscopy techniques are combined with rheological measurements to assess the influence of polymer end groups (–Cl or –OH) and water injection on the HNTs dispersion state. A morphological transition form microcomposite to nanocomposite is achieved when replacing –Cl chain ends of PES by –OH groups, especially when water is injected during processing. By a combination of Soxhlet extraction and thermogravimetric analysis, we show that some PES(OH) chains are covalently bonded onto the aluminosilicate surface during extrusion. A mechanism describing the physico-chemical action of water is presented. The best system in terms of clay dispersion has been retained to characterize PES-HNTs nanocomposites with respect to their thermo-mechanical, thermal and fire (mass loss calorimetry and UL-94) properties. Dynamic mechanical analysis shows a significant enhancement in the storage modulus of halloysite-based nanocomposites when compared to the unfilled matrix. The improved thermal and thermo-oxidative stability of PES in presence of HNTs is mainly attributed to the labyrinth effect provided by individually dispersed nanotubes, which is reinforced during the decomposition process by the formation of a protective charred ceramic surface layer. The mechanism of action of HNTs for fire retardancy of PES presumably arises from a synergistic effect between physical (i.e. ceramic-like structure formation and mechanical reinforcement of the intumescent char) and chemical (i.e. charring promotion) processes taking place in the condensed phase. According to this study, the straightforward and cost-effective melt compounding route could pave the way for future development of high-performance nanoscale polymeric materials combining enhanced thermal properties and excellent flame retardant behaviour.