Thermal properties of liquid crystalline aromatic random copolyesters containing two different mesogenic modifications within a polymer backbone

Two types of semiflexible mesogenic random copolyesters which contained both nematic and smectic type repeating units within the main chain were prepared and the thermal properties as well as the mesomorphic structures were characterized by differential scanning calorimetry and by polarizing microscope. The first type of copolymer (copolymer I) contained two repeating units which differed in the rigid mesogenic groups and the second (copolymer II) contained repeating units which differed in the length of flexible segment. Copolymerization disrupted the structural regularity of the crystal, lowered the crystal–mesophase transition temperature, and destabilized the molecular order of the smectic phase. These effects were more pronounced for copolymer II. However, the isotropization temperature was changed only slightly with the incorporation of the other mesogenic component. The crystal melting temperatures of both copolymers exhibited a eutectic behavior. A smectic to nematic transition, which was not observed for the homopolymers, occurred in the range of 0.4-0.6 mole fractions of the smectic units for copolymer I and 0.5–0.85 for copolymer II. The molecular order of the nematic phase was slightly increased as the smectic units were incorporated, while those of the smectic phase became more disordered upon addition of the nematic units.     

Tensile properties of plant fibre‐polymer composites in fire

The structural performance of polymer composites reinforced with plant fibres when exposed to fire was experimentally evaluated and compared against an E‐glass fibre laminate. Fire testing under combined one‐sided radiant heating and static tensile loading revealed that flax, jute, or hemp fibre composites experience more rapid thermal softening and fail within much shorter times than the fibreglass laminate, which is indicative of vastly inferior structural performance in fire. The plant fibre composites soften and fail before the onset of thermal decomposition of the plant fibres and polymer matrix, whereas the E‐glass fibres provide the composite with superior tensile properties to higher temperatures and higher applied tensile stresses. The tensile performance of the three types of plant fibre composites in fire was not identical. When exposed to the same radiant heat flux, the flax fibre composite could withstand higher tensile stresses for longer times than the hemp and jute laminates, which showed similar performance.     

Mixtures of poly(ethylene terephthalate) and liquid-crystalline polyesters with Y-shaped mesogens

Some liquid-crystalline polyesters (LCP) with Y-shaped mesogens, differing only in the length of their spacers, were blended with poly(ethylene terephthalate) (PET). Calorimetric measurements show significant influence of the LCP on the crystallization of PET even at low concentrations. The mixtures are spun into fibres and their mechanical properties are compared with fibres made from pure LCPs. The formation of LCP-fibrils leads to improved values of Young's modulus and tensile strength.     

Properties of a semi-crystalline and an amorphous thermotropic liquid crystalline polymer

The physical properties, thermal stability, rheology and tensile properties of a commercial semi-crystalline and an amorphous thermotropic liquid crystalline polymer (TLCP) have been investigated. Analysis by differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA) confirm the presence of a small melting endotherm and a glass transition in the former material. The as-received amorphous TLCP exhibits no obvious melting endotherm and a strong glass transition is detected. The flow and tensile properties of the semicrystalline polymer are dominated by the presence of the crystalline to nematic transition temperature. The properties of the amorphous TLCP appear to be governed by increasing mobility afforded by increasing temperature. Based on flow behaviour and further DSC analysis it has been shown that under appropriate annealing conditions the as-received amorphous TLCP can develop solid crystalline order.     

Flax fibre physical structure and its effect on composite properties: Impact strength and thermo-mechanical properties

Earlier investigations by the authors showed that the tensile modulus of flax fibre mat polypropylene composites (NMT) could surpass the values of glass mat reinforced thermoplastic (GMT) on fibre weight basis. The tensile and flexural strength could reach values of up to 65% of the GMT strength values, however, very much dependent on the fibre physical structure. This study deals with the Charpy impact and the thermo-mechanical properties of flax NMT materials. The trend is that the Charpy impact strength decreases with increasing fibre internal bonding and enhanced fibre-matrix adhesion, which is opposite to the trend for the tensile and flexural properties. The impact strength of the NMT materials is lower than generally reported for GMT materials. Dynamic mechanical thermal analysis reveals that with increasing temperature the storage modulus of the NMT materials reduces more slowly when the fibre internal bonding and the fibre-matrix adhesion are improved. In order to approach the tensile, flexural and impact strength of GMT materials, composites should be based on the strong elementary flax fibres. The axial tensile strength of elementary fibres approaches the strength of glass fibres and the lateral strength of the elementary fibres is higher than the technical flax fibres lateral strength. The thermo-mechanical properties can probably be improved when non-cellulosic material can be removed from the flax fibre surface without damaging the fibre.     

Some physical properties of acrylamide and acrylonitrile grafted nylon 6

Graft copolymerization of acrylamide and acrylonitrile on nylon 6 swollen in formic acid (60%) for 30 min was performed using ceric ions as initiator at 60°C. The optimum conditions to get highest percentage of graft-on were utilised to prepare the grafted samples with various amounts of polymer deposited inside the fibres. The mechanical properties, moisture regain, density, infrared studies, birefringence, dye uptake and dynamic mechanical properties were determined for the grafted samples and the results are discussed in relation to possible structural changes due to graft copolymerisation.     

Properties,morphology and structure of BPDA/PPD/ODA polyimide fibres

Abstract

A family of random co-poly(amic acid)s containing 4,4′-oxydianiline (ODA) moiety were synthesised in N,N′-dimethylacetamide. The co-poly(amic acid) solutions were used as spinning dope for dry jet wet spinning process into as spun poly(amic acid) (PAA) fibres. The polyimide (PI) fibres were obtained from PAA fibres after being imidised and drawn in furnace. The processability and mechanical properties of the fibres were notably improved by incorporating ODA into 3,3′,4,4′-biphenyltetracarboxylic dianhydride/p-phenylenediamine (BPDA/PPD) backbone. The best strength and modulus of BPDA/PPD/ODA PI fibre (diamine mole ratio of PPD/ODA?=?85∶15) attained 2·25 and 96·5 GPa respectively, which were approximately three times the tenacity of the BPDA/PPD PI fibre. The SEM image showed that the cross-section of each stage fibres was round and void free. In addition, ‘skin–core’ and microfibrillar structure were not observed. The thermal properties of PI fibres were also investigated. The results showed that the PI fibres have excellent thermal stability; moreover, the dimensional stability and structural homogeneity of the fibres were significantly improved by heat drawn stage. T_g was found to be ～290°C by thermomechanical and dynamic mechanical analyses. The X-ray (wide angle X-ray diffraction and small angle X-ray scattering) experiments indicated that the ordering degree of longitudinal and lateral stacks, as well as the molecular orientation of PI fibre, was improved in the preparation process of fibres. Furthermore, the mechanical properties of fibres are profoundly affected by the heat drawn conditions.     

Side-Chain Liquid Crystalline Polymers with [α-terpineol-co-MMA] Main Chain: Synthesis and Characterization of Polymers with Phenyl Benzoate Mesogenic Group

Summary Side chain liquid crystalline (SCLC) [α-terpineol-co-MMA] polymers with a phenyl benzoate mesogenic group with polymethylene spacers have been synthesized and characterized in which the spacer length is taken 9 methylene units. The thermal behavior and liquid crystallinity of the polymer has been characterized using Differential Scanning Calorimetry (DSC) and Polarized Optical Microscopy (POM) techniques . The DSC curve of the LCpolymer shows glass transition at 52 °C followed by nematic phase which undergoes isotropization at 120 °C without undergoing side chain crystallization. Under optical polarized microscope the appearance of characteristic schlieren texture confirms the presence of nematic phase .     

Effects of soft-segment prepolymer functionality on the thermal and mechanical properties of RIM copolymers

Segmented copolyureas (PUr) and copoly(urethane–urea)s (PUU) comprising 50% by weight of polyurea hard segments (HS) and polyether soft segment (SS) with different functionalities, have been formed by reaction injection moulding (RIM). The HS were formed from 4,4′-diphenylmethane diisocyanate reacted with mixed isomers of 3,5-diethyltoluene diamine. The nominal functionality of the SS prepolymers used (either amino- or hydroxyl-functionalised polyoxypropylenes with a constant molar mass per functional group of ∼ 2000 g mol⁻¹) was systematically increased from 2 to 4. RIM materials were characterised using dynamic mechanical thermal analysis, differential scanning calorimetry, tensile stress–strain and fracture mechanics studies. Generally, the PUr exhibited far superior thermal-mechanical properties than equivalent PUU materials but inferior fracture resistance, owing to morphological variations resulting from differences in copolymerisation behaviour. For both systems, tensile behaviour was shown to be dominated by the degree of phase separation, whereas fracture properties showed a degree of dependence on SS functionality.     

Liquid crystalline polyesters by staged-addition polycondensation

Summary A series of model liquid crystalline (LC) polyesters have been prepared by low temperature solution polycondensation using staged addition methods. The model thermotropic polyesters were composed of aliphatic spacers and mesogenic groups made from substituted terephthalic acid and methyl hydroquinone. Both the mesogenic groups and the spacers were placed in the main chain. Attractive properties of these polymers include solubility in many organic solvents such as THF and DMSO with softening temperatures below 150°C. Some of these polymers show no melting transitions, and depending on the terephthalate group substituent, either smectic or nematic phases are formed. Results from GPC analysis show good solubility of polymers in THF with M_w lower than 10,000 compared to polystyrene standards. The synthesis and characterization of these LC polyesters are reported here.     

Thermal and Mechanical Properties of Flax Fibres as Potential Composite Reinforcement

Scutched and hackled long flax fibres varying in retting degree, namely green, under‐retted and normally retted flax, and elementary fibres were tested for their mechanical and thermal properties. Tensile tests were performed on individual fibres and on bundles. The thermal stability is evaluated with thermogravimetric analyses (TGA) and determination of the mechanical properties after thermal exposure. For the investigated samples, no significant influence of the retting degree on tensile strength can be determined. Exposure to different temperatures results in a weight decrease and affects the mechanical properties of the fibres.     

Analysis of the linear methods for determining copolymerization reactivity ratios,VIII. A critical reexamination of radical copolymerization of methylmethacrylate or styrene

The reactivity ratios of selected methylmethacrylate or styrene copolymerisation systems have been reviewed in terms of the extended Kelen-Tüd?s method. r-values, together with their 95% confidence limits have been calculated. Quantities δ and Q, suitable for classification of the systems, have been applied. Of the 71 reevaluated methylmethacrylate systems 40 (56.3%) were found to belong to class I and 20 (28.2%) to class I(!), i. e. 60 (84.5%) systems for which the conventional copolymerisation equation was found to be adequate. For 3 (4.2%) systems belonging to class II the two-parameter model does not hold and 8 (11.3%) systems were related to class III, i. e. systems for which the experimental data are inconsistent and the published r-values meaningless. 32 styrene copolymerisation systems were reevaluated with the following classification: 19 (59.4%) systems in class I, 5 (15.6%) systems in class I(!), 2 systems in class II and 6 (18.8%) in class III.     

Morphology and properties of blends of polyethylene with a semiflexible liquid crystalline polymer

Blends of three polyethylene (PE) samples (two HDPE grades and LLDPE) with an experimental sample of a semiflexible liquid crystalline polymer (SBH 1:1:2 by Eniricerche) have been prepared in a Brabender compounder. The processing-aid effect of the LCP has been demonstrated by the decreased energy required for extruding the blends, as compared to that needed for neat PE. The thermal properties, as studied by differential scanning calorimetry (DSC), have shown that the two components of the blends are immiscible. However, the dispersed SBH phase has been found to act as a nucleating agent for the crystallization of LLDPE, whereas no such effect was observed for HDPE. This has been taken as an indication that the phase interactions of SBH with LLDPE are more pronounced than with HDPE. The morphological study of the blends, done by scanning electron microscopy (SEM), has confirmed this conclusion. In fact, the SBH particles show a much better dispersion and a narrower size distribution in the LLDPE/SBH blends. The mechanical properties of the blends have been studied on compression-molded specimens. The results indicate that the reinforcing effect of SBH is practically none for both HDPE grades. In fact, the elongation at break decreases to very low values, and the tensile strength is also reduced, when the LCP concentration increases beyond 5–10%, whereas the tensile modulus does not vary appreciably, over the whole (0–20%) LCP range investigated. On the contrary, the tensile modulus of the LLDPE/SBH blends increases up to ca. 50%, and the elongation at break decreases more smoothly, on increasing the SBH content up to 20%. © 1995 John Wiley & Sons, Inc.     

Preparation and characterisation of luminous polypropylene/phosphor strontium aluminate nanocomposite fibres with enhanced dyeability

Phosphor strontium aluminate (SrAl₂O₄: Eu²⁺, Dy³⁺) nanoparticles, SAOED, were melt compounded with polypropylene on an internal mixer to prepare a uniform mixture with polymer and the resulting materials were granulated. PP/SAOED nanocomposite fibres were then produced by melt spinning method at the take-up speed of 1000?m?min^?1. Morphological, thermal, structural and luminescent properties of the luminous fibres were studied by SEM, differential scanning calorimetry (DSC), wide angle X-ray diffraction (WAXD) and fluorescence spectrophotometer. The luminous fibres were then knitted and dyed to investigate the effects of SAOED nanoparticles on the dyeability of fibres. SEM results showed that with increasing SAOED nanoparticle amounts, the agglomerates of SAOED nanoparticles were appeared on the fibre surface. DSC results indicated that the SAOED nanoparticles did not affect melting and crystallisation behaviours significantly. WAXD patterns revealed that the crystal type of luminous nanocomposite fibres did not change and was still the α-monoclinic crystal form. The luminous nanocomposite fibres were found to have inferior tensile properties as compared to the neat PP fibre. From fluorescence spectrophotometer results, it can be seen that fibres excited at the wavelength of 400?nm, showed an emission peak at 550?nm and had green light. With increasing the amount of SAOED nanoparticles in the fibres, emission peak intensity of luminous fibres increased and also the fabric dyeability was improved.     

Simultaneous polymerization and molding of pyrrone polymers

Our previously reported one-step method of simultaneously polymerizing and molding pyrrone monomers to produce high-strength, high-modulus, dense, thermally stable moldings has been studied in more detail and improved by using crystalline 1 : 1 salts of the monomers. Interrelationships between molding powder characteristics and molding conditions required to achieve optimum polymer properties were investigated. Typical properties of the moldings obtained from the salt of pyromellitic anhydride and 3,3′-diaminobenzidine were: density, 1.29 g/cc; diametral tensile strength, 10,700 psi; ultrasonic modulus, 1.3 × 10⁶ psi; hardness, 80 Rockwell B; and thermal stability in air (at 0.5°C/min), 400°C. Quick TGA methods, applied to the salts for estimating the thermal stability of molded polymers, before actual polymerization and molding, have been developed. No problems associated with gas release are observed when molding conditions have been optimized.     

The modification of wild silk fibres with methacrylamide

Wild silk and cultivated mulberry silk fibres have been graft copolymerised with methacrylamide, and the changes in the dyeing behaviour when dyeing with the levelling acid and milling acid dyes investigated. The tensile properties of the fibres and their durability during laundering have also been examined. The amount of the levelling acid dye adsorbed by all the fibres decreased with increasing polymer add-on, while the amount of milling acid dye adsorbed by the mulberry silk fibres was almost unchanged by the graft treatment. The breaking loads of the fibres were almost unchanged whereas rigidity of the fibres increased following graft treatment. The tensile properties of the grafted silk fibres were not diminished by laundering at polymer add-on levels greater than 50%.     

Synthesis and mesogenic behavior of metal-containing liquid crystalline networks

A new hydroxy functionalized liquid crystalline (LC) polyazomethine has been synthesized by the solution polycondensation of a dialdehyde with a diamine. The polymer was characterized by IR, ¹H-, and ¹³C-NMR spectroscopy. Studies on the liquid crystalline properties reveal the nematic mesomorphic behavior. This polymer functions as a polymeric chelate and forms a three-dimensional network structure through the metal complexation. Influence of various metals and their concentration on the liquid crystalline behavior of the network has been studied. Networks up to 30 mol % of the metal show LC phase transitions; above this the transitions are suppressed and the network behaves like an LC thermoset. © 1996 John Wiley & Sons, Inc.     

Polytrifluormethylstyrole,eine polymerklasse mit aussergewöhnlichen optischen eigenschaften

Homopolymers of 2-, 3- and 4-trifluoromethylstyrene, 4-perfluoroisopropylstyrene, 3,5-bis-trifluoromethyl-styrene and a 1 : 1 copolymer of 2,4- and 2,5-bis-trifluoromethyl-styrene have been synthesized and the refractive indices n_D, Abbe-numbers ν_D and glass transition temperatures of the polymers have been determined. With regard to optical applications poly-2-trifluoromethyl-styrene is particularly interesting since its optical properties are in a region of the n-ν-diagram that has not been reached by thermoplastic polymers or inorganic glasses before. The most important mechanical and thermal properties of this polymer are reported. Reactivity ratios of the binary copolymerisation of 2-trifluoromethyl-styrene with styrene, methyl-methacrylate and n-butyl-acrylate are listed as well as optical data and glass transition temperatures of some copolymers of 2-trifluoromethyl-styrene.     

Tensile deformation of regenerated and native cellulose fibres

The tensile deformation of well-oriented cellulose fibres has been reexamined. It is demonstrated that not only the fibre structure but also the deformation mechanism is similar to that found in high performance aramid fibres. An explanation is proposed for the difference between the elastic moduli of cellulose I and II. In view of these results it is possible to assess the prospect for improving the mechanical properties of cellulose fibres.     

Characterisation of the microstructure and deformation of high modulus cellulose fibres

This paper reports on the microstructure and deformation of one type of high modulus cellulose fibre characterised using the techniques of Raman spectroscopy and synchrotron X-ray diffraction and it compares this fibre to a lower modulus conventional viscose fibre. The crystallinity of the fibres has been determined using X-ray diffraction. The orientation parameter has been determined by measuring the width of the (200) equatorial reflections for each fibre using microfocus synchrotron radiation and it has also been shown that the crystal orientation parameter varies from the skin to core of the fibres and is different for each type. Mechanical properties of the fibres are reported and it is shown that the high modulus cellulose fibres have very different stress-strain behaviour to the viscose fibres. Finally, it is shown for the high modulus fibre that the 1414, 1260, 1095 and 895 cm⁻¹ Raman bands shift under the application of tensile deformation towards a lower wavenumber with the 1095 cm⁻¹ band giving information about the backbone chain stretching of the cellulose. The viscose fibres show less significant shifts in this peak. The crystal modulus of the high modulus cellulose fibre has also been determined by calculating the change in the c-spacing upon the application of tensile deformation to individual cellulose monofilaments. This change in the c-spacing is determined from the change in position of the (002) meridional reflection giving a crystal modulus of 77 GPa. This value is a little low compared to other published data, and reasons for this are discussed. The shear modulus between crystallites is also calculated and compared to previously published data.