Thermal analysis studies on thermoplastic rigid rod molecular composites

Rigid rod molecular composites involve molecular level mixing of two polymers having greatly disparate chain conformations (one rodlike and one flexible coil like). Morphology studies have shown that in such materials the bundle diameter of these rigid rod molecules is less than 5 nm. The rigid rod molecule used is poly(para-phenylene benzobisthiazole) (PPBT). The thermoplastic flexible polymer matrix is Nylon-66, DuPont's Zytel 42. Thermal analysis studies on these materials indicate that the expected T_g shifts, as observed for blends of two flexible polymers, and depression of T_m with increased rod molecule composition does not occur.     

Nematic-like mesophase photoconductive polymer for photorefractive applications

A new nematic-like mesophase photoconductive polymer PPT-TPA consisting of wholly aromatic rigid backbone of poly(p-phenyleneterephthalate), PPT and pendent hole-transporting triphenylamine (TPA) groups attached to the ends of oxydecyl spacers has been synthesized. The photorefractive composite contains the photoconductor PPT-TPA, the chromophore diethylaminodicyanostyrene (DDCST), and the photosensitizer C₆₀. Although no plasticizer was added, the glass transition temperature T_g of the composite is 15 °C, which characterizes it as a low-T_g photorefractive material. We investigate the correlation between the mesophase structure and its optical/physical properties by X-ray diffraction, photoconductive and photorefractive experiments. The new composite and its properties are compared to PPT-CZ composites with only a different charge transporting agent (carbazole, CZ) but a much more ordered mesophase structure, which were studied previously and have shown very good photorefractive properties. Despite of a lower photoconductivity of the new photorefractive composite PPT-TPA (n=10):DDCST:C₆₀ this material shows a higher photorefractive sensitivity S_n2 of 2±0.2 cm²/kJ at E=50 V/μm than the previously synthesized composite PPT-CZ (n=10):DDCST:C₆₀.     

Microstructural modifications in uniaxially hot‐drawn polycyclohexylene terephthalate films

Semi‐aromatic thermoplastic polycyclohexylene terephthalate (PCT), initially wholly amorphous, was uniaxially drawn to study microstructural modifications as the appearance of the strain‐induced (S.I.) crystalline phase. Polyethylene terephthalate (PET) and poly(ethylene glycol‐co‐cyclohexane‐1,4‐dimethanol terephthalate) (PETG) are considered as reference materials in this work. In polycyclohexylene terephthalate (PCT) the presence of a saturated ring (which is not quite as rigid as the aromatic ring) modifies the characteristics of both thermal and S.I. crystallization. Samples with various draw ratios (drawing of PCT films is performed at T > T_g) were analyzed by Modulated Differential Scanning Calorimetry, wide angle X‐ray scattering, and birefringence measurements. In drawn PCT films, an S.I. crystalline phase appears continuously with the draw ratio and reaches 35%. For this polymer and for the highest draw ratio, the “true” amorphous fraction practically disappears. The material is composed only of the S.I. crystalline phase and the “rigid” amorphous phase. Polym. Eng. Sci. 44:509–517, 2004. © 2004 Society of Plastics Engineers.     

Enhancement of orientation of rigid polymer blocks by incorporating rod–coil block copolymer chains into metal–organic frameworks

Incorporation of polymer chains into metal–organic frameworks (MOFs) is a simple yet efficient method for improving the orientation of the polymer chains. However, due to their rigidity and high molecular weight, many rigid polymer chains are either not easily loaded into MOFs, or not easily aligned within the MOF channels. In this paper, we propose a strategy for enhancing the orientation of rigid blocks by incorporating rod–coil block copolymer chains into MOFs. In this strategy, on the one hand, the rigid blocks have a low molecular weight, so that the steric hindrance effect from the MOF channels could align the rigid blocks more efficiently. On the other hand, because the covalent bonds between the repeat units of the flexible blocks can rotate relatively easily, the steric hindrance effect from the flexible blocks can further help the rigid blocks to be oriented within the MOF channels. In confirmatory simulations, two all‐atom MOF models, [Zn₂(BDC)₂(TED)]_n and [Zn₂(BPDC)₂(TED)]_n (TED = triethylenediamine, BDC = 1,4‐benzenedicarboxylate, BPDC = 4,4′‐biphenyldicarboxylate), are established. With these MOF models, molecular dynamics simulations are performed for the rigid poly(phenylene vinylene) (PPV) chain and the rod–coil block copolymer PPV‐block‐polystyrene (PSt). Further, their respective degrees of orientation (DoOs) are calculated. Within [Zn₂(BDC)₂(TED)]_n and [Zn₂(BPDC)₂(TED)]_n, the DoOs of the rigid PPV blocks of PPV‐block‐PSt are 0.968 and 0.902, respectively, while the DoOs of the rigid PPV chains are 0.865 and 0.711, respectively. These calculation results prove the feasibility of our proposed strategy. © 2019 Society of Chemical Industry     

Characterization and free radical polymerization of liquid crystalline acrylic acid/cellulose diacetate and N-vinyl-2-pyrrolidinone/cellulose diacetate solutions

Polymerizations of liquid crystalline solutions of cellulose diacetate (CDA) in acrylic acid (AA) and N-vinyl-2-pyrrolidinone (NVP) were conducted in an attempt to prepare molecular composites (polymer blends) processing a rigid rod polymer with liquid crystalline orientation. CDA was found to form liquid crystalline solutions in both AA and NVP at concentrations avove 40 wt% CDA. Polymerization of anisotropic 50 wt% CDA-AA and CDA-NVP solutions occurred with considerable retention of the starting solution anisotropy and yielded homogeneous blends (1 T_g) when the rate of polymerization was fast relative to the phase separation of the free radically polymerizing AA or NVP with CDA. Slow polymerizations lead to phase separated blends (2 T_g).     

Modification of nylon-6 with wholly rigid poly(m-phenylene isophthalamide)

In this study, the flexible nylon-6 was reinforced by the wholly rigid aromatic polyamide poly(m-phenylene isophthalamide) (PmIA) (Nomex) by physical polyblending and chemical copolymerization using p-aminophenylacetic acid (P-APA) as a coupling agent. From DSC measurements, it was shown that T_g of the polyblends increased with the increase of Nomex content. The T_g and T_m of multiblock copolyamides were found to be higher than those of polyblends and triblock copolyamides. Scanning electron microscopy revealed that the polyblends were a dispersed phase structure, although the multiblock copolyamides exhibited a homogeneous texture rather than an aggregated one. From the wide-angle X-ray diffraction pattern, it was found that the triblock copolyamides and polyblends had two diffraction peaks, i.e., 2 θ = 20.5 and 24°. However, the multiblock had only one at 2 θ = 20°, indicating a different crystal structure for multiblock copolyamides. For the mechanical properties, it was found that the multiblock copolyamides had a more significant reinforcing effect than those of polyblends and triblock copolyamides.     

Thermal analysis of polymer ignition

Gas phase criteria for the onset of flaming combustion of solids in fires are used to locate a critical temperature T_cr in a nonisothermal analysis (TA) experiment that corresponds to the surface temperature of the solid at ignition in a fire test, T_ign. This critical TA temperature occurs at low conversion of solid to gaseous fuel so it is independent of the heating rate in the test or the thermal decomposition reaction model. However, T_cr depends on the thermal properties of the polymer and the conditions of the fire test in which the gas phase criteria were measured. Nonisothermal analysis data in nitrogen and air were obtained for 20 polymers by thermogravimetric analysis and microscale combustion calorimetry. The critical temperatures T_crs obtained from TA experiments compared favorably with analytic results for a simple polymer ignition model and finite element simulations and were in qualitative agreement with ignition temperatures measured in standardized fire tests.     

Thermal behavior of aliphatic–aromatic poly(ether-amide)s

The thermal properties of a set of experimental aliphatic–aromatic polyamides containing ether linkages were examined as a function of their chemical structure. Variations of the glass transition temperature (T_g) and melting temperature (T_m) could be correlated with the length of the aliphatic spacers and with the orientation of the phenylene rings. Polymers with a high concentration of p-oriented phenylene units showed a higher T_g than those containing mainly m-oriented ones; T_g values ranged from 110 to 155°C. Surprisingly, a negligible dependence of T_gs on the nature of flexible spacers was observed. For all of the polymers, the thermal stability was virtually the same, about 440°C, when tested by dynamic thermogravimetric analysis (TGA). However, quite different levels of thermal stability were found by isothermal TGA analysis for polyamides with different flexible spacers. Moreover, the poly(ether-amide)s described here compare fairly well with wholly aromatic polyamides when measured by dynamic TGA; but isothermal TGA measurements clearly demonstrated that they decompose faster than aromatic polyamides. Treatment of the TGA curves by the method of McCallum provided kinetic data that confirmed a better long-term stability for poly(ether-amide)s with a higher proportion of para-oriented phenylene rings. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67:975–981, 1998     

The totally miscible in ternary hydrogen‐bonded polymer blend of poly(vinyl phenol)/phenoxy/phenolic

The individual binary polymer blends of phenolic/phenoxy, phenolic/poly(vinyl phenol) (PVPh), and phenoxy/PVPh have specific interaction through intermolecular hydrogen bonding of hydroxyl–hydroxyl group to form homogeneous miscible phase. In addition, the miscibility and hydrogen bonding behaviors of ternary hydrogen bond blends of phenolic/phenoxy/PVPh were investigated by using differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy, and optical microscopy. According to the DSC analysis, every composition of the ternary blend shows single glass transition temperature (T_g), indicating that this ternary hydrogen‐bonded blend is totally miscible. The interassociation equilibrium constant between each binary blend was calculated from the appropriate model compounds. The interassociation equilibrium constant (K_A) of each individually binary blend is higher than any self‐association equilibrium constant (K_B), resulting in the hydroxyl group tending to form interassociation hydrogen bond. Photographs of optical microscopy show this ternary blend possess lower critical solution temperature (LCST) phase diagram. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Ternary polymer mixtures: Polyarylate/phenoxy/poly(butylene terephthalate)

The intended objective of this work was to bring together two immiscible polymers, polyarylate (PAr) and Phenoxy [poly(hydroxy ether of bisphenol-A)], preparing ternary mixtures with a third component, poly(butylene terephthalate) (PBT). Experimental results showed that ternary mixtures containing 30% or more PBT gave single glass transition temperatures by DSC. Moreover, the PBT melting point depended on the composition of the mixtures. These results, which could be indicative of the existence of a single amorphous phase in these blends, have been discussed. Nevertheless, results must be considered with caution, given the peculiarities of the T_g–composition diagrams for the miscible pairs PAr/PBT and Phenoxy/PBT. Hypothetic interchange reactions during melting have been found to be unimportant.     

Flexible all‐aromatic polyesterimide films with high glass transition temperatures

A series all‐aromatic poly(esterimide)s with different molar ratios of N‐(3′‐hydroxyphenyl)‐trimellitimide (IM) and 4‐hydroxybenzoic acid (HBA) (IM/HBA = 0.3/0.7 and 0.7/0.3) was prepared with the aim to design flexible high T_g films. Melt‐pressed films, either from high molecular weight polymer or cured phenylethynyl precursor oligomers, exhibit T_gs in the range of 200 °C to 242 °C and are brittle. After a thermal stretching procedure, the films became remarkably flexible and very easy to handle. In addition, the thermally stretched 3‐IM/7‐HBA and 7‐IM/3‐HBA films show tensile strengths of 108 MPa and 169 MPa, respectively. Thermal treatment increased the T_g of 3‐IM/7‐HBA from 205 °C to 248 °C, whereas the T_g of 7‐IM/3‐HBA increased from 230 °C to 260 °C. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 133, 44774.     

Modification of aromatic diamine-cured epoxy resins by poly(oxymethylene) or hybrid modifiers containing poly(oxymethylene)

A copolymer comprising poly(oxymethylene) (POM, polyacetal) was used to improve the fracture toughness of a resin based on diglycidyl ether of bisphenol A (DGEBA) cured with 3,3′-dimethyl-5,5′-diethyl-4,4′-diaminodiphenyl methane. POM was a less effective modifier for epoxies and a third component was used as a toughener or a compatibilizer for POM. The third component includes polypropylene glycol-type urethane prepolymer (PU) and aromatic polyesters. The hybrid modifiers composed of POM and PU were more effective as modifiers for toughening epoxies than POM alone. In the ternary DGEBA/POM/PU (90/10/10wt ratio) blend, the fracture toughness, K_IC, for the modified resin increased 50% with retention of flexural properties and a slight decrease in glass transition temperature (T_g) compared with those of the unmodified epoxy resin. The aromatic polyesters include poly(ethylene phthalate) (PEP), the related copolyesters and poly(butylene phthalate). PEP was most effective of them as a third component in the hybrid modifier. In the ternary DGEBA/POM/PEP (85/15/10) blend, K_IC for the modified resin increased 70% with medium loss of flexural strength and retention of T_g. The toughening mechanism is discussed in terms of morphological and dynamic viscoelastic behaviour of the modified epoxy resin systems. ©1997 SCI     

Miscibility windows in ternary polymer blends of a polyester,polymethacrylate and poly(styrene‐co‐acrylonitrile)

Miscibility, phase diagrams and morphology of poly(ε‐caprolactone) (PCL)/poly(benzyl methacrylate) (PBzMA)/poly(styrene‐co‐acrylonitrile) (SAN) ternary blends were investigated by differential scanning calorimetry (DSC), optical microscopy (OM), and scanning electron microscopy (SEM). The miscibility window of PCL/PBzMA/SAN ternary blends is influenced by the acrylonitrile (AN) content in the SAN copolymers. At ambient temperature, the ternary polymer blend is completely miscible within a closed‐loop miscibility window. DSC showed only one glass transition temperature (T_g) for PCL/PBzMA/SAN‐17 and PCL/PBzMA/SAN‐25 ternary blends; furthermore, OM and SEM results showed that PCL/PBzMA/SAN‐17 and PCL/PBzMA/SAN‐25 were homogeneous for any composition of the ternary phase diagram. Hence, it demonstrated that miscibility exists for PCL/PBzMA/SAN‐17 and PCL/PBzMA/SAN‐25 ternary blends, but that the ternary system becomes phase‐separated outside these AN contents. Copyright © 2003 Society of Chemical Industry     

Side chain dynamics in semiconducting polymer MEH-PPV

The characteristic nanoscale dynamics of the alkyl side groups in the light-emitting polymer poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] have been investigated using quasi-elastic neutron scattering (QENS). The measurements were taken below the polymer's glass transition (T ≤ T_g ≃ 353 K), where the main backbone is in a rigid state and does not contribute to the broadening of the QENS signal. An analytical diffusion model consisting of a static term and two dynamical components, characterizing the flexible side groups, provide an excellent fit to the experimental data. The two observed dynamical processes are all localized in character, with no meaningful dependence on temperature. The faster process, with characteristic timescale of ∼18 ps at room temperature (RT), can be linked to the average mobility of the terminal protons of the alkyl chain, while the slower process, with characteristic timescale of ∼170 ps at RT, to those protons at the other end of the alkyl chain, closest to the backbone. While the fraction of mobile protons contributing to the QENS signal increases with increasing temperature, the characteristic timescale and confining volume within which the protons are able to move locally depend chiefly on the polymer conformational state. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47394.     

Film formation from polymer dispersions

The coalescence of a poly(vinylidene chloride/methyl acrylate/acrylic acid) latex was found to occur only above the T_g. The various theories of film formation from polymer dispersions attribute latex coalescence to three types of forces: the pressure due to the polymer–air interface, the pressure due to a polymer–water interface, and the pressure due to capillary action. Analysis of the forces indicates that none are sufficient to cause coalescence below T_g while any, or all, may cause coalescence above T_g.     

Effect of chain rigidity on block copolymer micelle formation and dissolution as observed by 1H-NMR spectroscopy

Amphiphilic copolymers poly(methyl methacrylate-b-acrylic acid), poly(methyl methacrylate-b-methacrylic acid), poly(methyl acrylate-b-acrylic acid) and poly(methyl acrylate-b-methacrylic acid) were prepared by reversible addition fragmentation chain-transfer (RAFT) polymerization. The hydrophilic polyacid blocks were either synthesized directly or formed by the hydrolysis of poly(tert-butyl acrylate) or poly(tert-butyl methacrylate) blocks. The hydrophobic blocks consisted of either the more rigid, high glass transition temperature (T _g ) poly(methyl methacrylate) or more flexible, low T _g poly(methyl acrylate) material. The hydrophilic blocks were either poly(methacrylic acid) (rigid, high T _g ) or poly(acrylic acid) (flexible, low T _g ). The micellization behavior of the polymers was studied by proton nuclear magnetic resonance (¹H-NMR) spectroscopy in mixtures of 1,4-dioxane-d₈ and D₂O. All four polymers were soluble in neat dioxane. In solutions of higher water content, the polymers with the more rigid hydrophobic blocks formed into micelles as was evidenced by broadening of the resonances resulting from the protons in those blocks. At moderate water concentration (25–50%), dissolution of the micelles was observed upon heating the solution. No micellization was observed in polymers containing the less rigid poly(methyl acrylate) hydrophobic block regardless of the identity of the hydrophilic block. As further evidence of micellization formation and dissolution, the spin-lattice (T ₁) and spin-spin (T ₂) relaxation times of protons in the hydrophobic and hydrophilic blocks were measured. Significant differences in the relaxation times as functions of temperature and solvent concentration were observed between the hydrophilic and hydrophobic blocks of the micelle-forming polymers.     

Improvement of the processability of poly(ether ketone ketone) by the addition of a thermotropic liquid crystalline polymer

The addition of a thermotropic liquid crystalline, wholly aromatic copolyester, TLCP, improved the melt processability of poly(ether ketone ketone), PEKK. The tensile strength and modulus of the blends also improved with increasing TLCP, but the elongation at break decreased significantly. The blends were phase‐separated, but the polymers were partially miscible as evident from shifts of the glass transition temperature (T_g) of each component towards that of the other component in the blend. Similarly, the melting points (T_m) of both components were depressed by blending. When the crystallization temperature was above T_m of the TLCP, the PEKK crystallization rate in the blend was slower than for the pure material, while crystallization was faster when the temperature was below T_m of the TLCP. Polym. Eng. Sci. 44:541–547, 2004. © 2004 Society of Plastics Engineers.     

Aromatic polyamides. V. Substituent effect on thermal properties

Some wholly aromatic polyamides derived from unsubstituted and chloro- and nitro-substituted diamines have been studied from the viewpoint of their thermal stability, thermo-oxidative stability, and thermal transitions. General relationships between thermal stability of a polymer and its chemical structure are described. Decrease in thermal stability of poly(1,3-phenyleneisophthalamide) and poly(1,4-phenyleneterephthalamide) due to substituents has been explained and supported in part by infrared spectral data. The effect of electron-withdrawing substituents such as chloro and nitro in increasing the thermo-oxidative resistance of the polyamides is pointed out. The thermal transitions (T_g and T_m) of these polymers are also reported. All the polyamides exhibit a broad exothermic peak in the 630–700°C temperature range, which probably corresponds to reactions (crosslinking and cyclization) responsible for the high char yield of these systems.     

Miscible polymer blends containing poly(vinyl chloride)

Polymer blends have received particular interest in the past several decades in both industrial and academic research. An initial survey of miscible polymer pairs (1) (1968) revealed 12 combinations. A later survey (2) (1979) noted approximately 180 miscible pairs. Today possibly over 500 miscible combinations have been noted in the open and patent literature (3). However, the vast majority of possible polymer blend combinations are not miscible (thus phase separated). A significant number of diverse polymer structures have been shown to exhibit miscibility with PVC. Several of these blends have been studied in detail and have shown specific interactions primarily involving the α-hydrogen and PVC (considered the proton donor in proton donor-proton acceptor hydrogen bonding type interactions). The blend of poly(?-caprolactone) with PVC illustrates this interaction and has been reported in many published papers. While polymer miscibility in PVC blends offers significant academic interest, industrial utility is also of considerable importance. The addition of low T_g, miscible polymers to PVC offers permanent plasticization. The addition of high T_g, miscible polymers to PVC yields the desired heat distortion temperature enhancement of rigid PVC. A specific example of permanent plasticization involves nitrile rubber blends which have been commercial since the early 1940's. This presentation will review the growing number of polymers noted to be miscible with PVC. The importance of specific interactions will be discussed.     

Phototackification of polymer blends

A direct single-layer phototackification scheme is demonstrated making use of chemical amplification and photo-induced microphase separation in an initially nontacky miscible blend of acid-labile poly(2-tetrahydropyranyl methacrylate) and poly(2-tetrahydropyranyl acrylate), tacky poly(2-phenylethyl acrylate), and a photoacid generator. The mechanism involves four process: photoacid generation, acid migration, acetal ester cleavage, and phase separation. Thin film in situ IR studies showed the rate of acid migration and acetal ester cleavage to be strongly dependent on the presence of ambient water and polymer matrix (T_g) effects. The rate of phase separation is affected by polymer molecular weight. A number of approaches to minimize humidity sensitivity are discussed. The system has been sensitized to both UV and near-IR radiation. © 1994 John Wiley & Sons, Inc.