Structure of polyamide 6 and poly (p-benzamide) in their rod-coil-rod triblock copolymers investigated with in situ wide angle X-ray diffraction

A series of rod-coil-rod triblock copolymers containing polycaprolactam (PA6) as the coil block and poly (p-benzamide) (PBA) as the rod block were synthesized by a two-step polycondensation reaction. Proton nuclear magnetic resonance (¹H NMR), UV-vis spectrophotometry (UVS) and differential scanning calorimetry (DSC) were performed to determine the fundamental molecular structure and thermal property of copolymers. UV-vis spectrophotometry results revealed that the content of PBA homopolymer increased with the block length (number of monomer) of PBA and reached a plateau value ranging from 22 to 40 monomers of PBA. The wide angle X-ray diffraction (WAXD) measurements indicated that the crystallization of PA6 blocks was strongly suppressed due to the stretching from rods after annealed at temperature above the melting point of PA6. Only a few imperfect crystals of PA6 existed in the samples with low volume fraction of PBA. Moreover, the variation of PA6 block length hardly affected the crystallinity of PBA, which was dominantly controlled by the block length of PBA as diffusion was the control step for PBA crystallization at the annealing temperature.     

Free volume sizes in intercalated polyamide 6/clay nanocomposites

The effect of incorporating modified clay into a polyamide 6 (PA6) matrix, on the free volume cavity sizes and the thermal and viscoelastic properties of the resulting nanocomposite, was studied with positron annihilation lifetime spectroscopy, differential scanning calorimetry and dynamic mechanical analysis. At low concentrations of clay the fraction of PA6 crystals melting close to 212 °C was increased, while the fraction of the α-form PA6 crystals, melting close to 222 °C, was reduced. At higher concentrations of clay, a crystal phase with increased thermal stability emerged. Addition of more than 19 wt% clay caused a reduction of the heat of fusion of PA6. An unexpected reduction of the ΔC_p at the glass transition, contradicting the measured reduction of the heat of fusion, was detected, indicating an altered mobility in the non-crystalline regions. The viscoelastic response of PA6/clay nanocomposites, as compared to unfilled PA6, pointed towards a changed mobility in the non-crystalline regions. At high concentrations of clay (>19 wt%) an increase of the free volume cavity diameter was observed, indicating a lower chain packing efficiency in the PA6/clay nanocomposites. The increased free volume sizes were present both above and below the glass transition temperature of PA6.     

Structure of syndiotactic propylene-ethylene copolymers: Effect of the presence of ethylene units on the structural transitions during plastic deformation and annealing of syndiotactic polypropylene

Syndiotactic propylene-ethylene copolymers have been synthesized with a single-center C_s-symmetric syndiospecific metallocene catalyst. A study of the effect of the presence of ethylene comonomeric units on the polymorphic behavior of syndiotactic polypropylene (sPP) and on the structural transitions occurring during stretching is reported. For copolymer samples with low ethylene contents, in the range 2-7 mol%, crystals of the helical form I, present in the melt-crystallized samples, transform into the trans-planar form III by stretching at high deformation. Form III transforms in part into the helical form II by releasing the tension, as it occurs for sPP. Samples with ethylene contents in the range 8-10 mol% are crystallized from the melt as a mixture of crystals of helical form I and form II. Both helical forms transform by stretching at low values of deformation (lower than 300%) into the trans-planar mesomorphic form, which transforms into the trans-planar form III by further stretching at higher deformations (higher than 500%). For these samples form III transforms back into the mesomorphic form, rather than into the helical forms, by releasing the tension. Unoriented samples of copolymers with ethylene content in the range 13-18 mol% are mainly crystallized in the helical form II, which transforms into the trans-planar mesomorphic form by stretching. Upon releasing the tension the mesomorphic form remains stable and no polymorphic transition is observed. The presence of ethylene comonomeric units stabilizes the trans-planar forms in fibers of the copolymer samples. This has been confirmed by the result that for high ethylene contents the trans-planar form III and the mesomorphic form do not transform in helical forms by annealing of fibers stretched at high deformations.     

Structure evolution of polyamide (11)’s crystalline phase under uniaxial stretching and increasing temperature

Thermal processing of polyamide influences the internal crystalline structure and thereafter the post product mechanical performance. In this article, the crystalline transition of polyamide-11 (PA11) plate under uniaxial stretching and increasing temperature was investigated systematically using in-situ synchrotron X-ray technique. It was discovered that the lamellar slippage, fragmentation and recrystallization occurred in sequence under increasing temperature. In detail, the crystal of PA11 plate was stretched with a transition from triclinic α-form to mesomorphic phase at 25 °C. For the thermally activated γ-form crystals, crystal transition was inhibited when temperature was increased up to 160 °C. The melt-recrystallization was inclined to take place at large tensile strains. This work enhances the research significance of the thermal processing of polyamide and provides a theoretical method to improve the high performance of polyamide products.     

Synthesis of imidazolium ABA triblock copolymers for electromechanical transducers

Nitroxide-mediated polymerization enabled the synthesis of cationic, imidazolium-containing triblock copolymers as a membrane for an electromechanical transducer. Nitroxide-mediated polymerization afforded poly(styrene-b-[1-ethyl-3-(4-vinylbenzyl)imidazolium bis(trifluoromethane sulfonyl)imide)]-b-styrene) in a controlled fashion as confirmed using aqueous size exclusion chromatography and ¹H NMR spectroscopy. Dynamic mechanical analysis revealed a modulus of approximately 100 MPa for the triblock copolymer at 23 °C, which was suitable for fabrication of an electromechanical actuator. Evaluation of electromechanical actuators revealed device curvatures over twice the curvatures for Nafion^® in both the presence and absence of a conductive network composite. Addition of the ionic liquid (IL) 1-ethyl-3-methylimidazolium trifluoromethane sulfonate selectively reduced the glass transition temperature (T_g) of the central block and increased overall ionic conductivity. Normalizing temperature with the central block T_g caused the ionic conductivity for the IL-incorporated polymers to collapse onto a single curve, which was an order of magnitude higher than the block copolymer in the absence of added IL.     

Structural effect of a series of block copolymers consisting of poly(N-isopropylacrylamide) and poly(N-hydroxyethylacrylamide) on thermoresponsive behavior

Well-defined diblock and triblock copolymers consisting of poly(N-isopropylacrylamide) (PNIPAM) and poly(N-hydroxyethylacrylamide) (PHEAA) were prepared using the atom transfer radical polymerization (ATRP) method. The number-average molecular weight and fraction of each segment were precisely controllable by adjusting the monomer/initiator ratio in feed. The lower critical solution temperature (LCST) of a series of block copolymers with different compositions was examined using a turbidimetry analysis. The copolymers with a relatively lower molar fraction of HEAA units in the polymer chain exhibited phase transition phenomenon, in which the LCST depended on the fraction in the copolymer. On the other hand, the LCST disappeared for the copolymers with higher HEAA unit molar fractions. The ¹H NMR measurement clarified that the disappearance of the LCST was attributed to the formation of the water-soluble micelle. Furthermore, the thermoresponsive property of the series of block copolymers was elucidated on the basis of the structural effect of the copolymer, which includes the order and length of the block segments.     

Polarized optical microscopy study on the superstructures of oxyethylene/oxybutylene block copolymers

The superstructures of oxyethylene/oxybutylene block copolymers with different compositions and architectures (E_mB_n, B_nE_mB_n and E_mB_nE_m) were studied using polarized optical microscopy (POM). Several novel superstructures, such as fibril crystals and different frontiers of spherulites and crystallized regions, have been observed. It is found that the ability of forming spherulites is reduced with the decrease in the volume fraction of the crystallizable block. The unfavorable interaction between the blocks, which can be indicated by order-disorder transition temperature (T_ODT), also affects the formation of superstructure. The B_nE_mB_n triblock copolymers exhibit the strongest ability of organization into spherulites, whereas the E_mB_nE_m triblock copolymers show the weakest ability of organization into spherulites.     

Self-assembly behavior of rod–coil–rod polypeptide block copolymers

Self-assembly behavior of rod–coil–rod poly(γ-benzyl-l-glutamate)-b-poly(ethylene glycol)-b-poly(γ-benzyl-l-glutamate) (PBLG-b-PEG-b-PBLG) triblock copolymers with various PBLG block lengths in aqueous solution was investigated. The PBLG-b-PEG-b-PBLG triblock copolymers are able to self-assemble into vesicles when PBLG block length is relatively short. Meanwhile, the initial polymer concentration was found to have influence on the self-assembly. Giant vesicles can be observed when the initial concentration is high. Dissipative particle dynamics (DPD) simulations about the vesicles revealed that the rigid rod blocks could be aligned parallelly with each other to form the monolayer vesicles wall. When the PBLG block length in the PBLG-b-PEG-b-PBLG triblock copolymers increases, the aggregate morphologies were observed to transform from vesicles to spherical micelles. Based on the experimental and simulation results, we proposed a possible mechanism of the morphological transitions of the rod–coil–rod triblock copolymer aggregates.     

Glass transition of undrawn and drawn copolyetherester thermoplastic elastomers

The phase and deformation behaviour of two types of copolyetheresters (the block copolymers E and P) were studied by means of DSC, dynamic mechanical spectroscopy and X-ray diffraction. The block copolymers E and P based on poly(butylene therephtalate) (PBT) as a hard block have poly(tetramethylene oxide) (PTMO) and triblock copolymer (PEO-PPO-PEO) with a middle poly(propylene oxide) (PPO) block and two end poly(ethylene oxide) (PEO) as a soft block, respectively. The complex investigation shows that the studied copolyetheresters are microphase separated polymer systems in the amorphous matrix of which the PBT crystallites are embedded. The volume fraction of PBT crystallites depends on the block copolymer composition and changes from 5 to 20%. In the amorphous matrix that is the mixed PBT/ the soft block phase the soft block acts as a PBT plasticizer reducing the glass transition temperature of the amorphous mixed phase. The most interesting aspect of the phase behaviour of the copolyetheresters consists in the fact that, in comparison with PTMO, the triblock is characterized by a more pronounced PBT plasticization effect. A special emphasis was placed in this work on the investigation of an increased glass transition temperature of drawn copolyetheresters. It was found that this process depends on the volume fraction of PBT crystallites and chemical structure of the soft blocks. The first factor characterizes the interaction between the amorphous mixed phase and PBT crystallites and, therefore, the higher value of the volume fraction of PBT crystallites the higher is the glass transition temperature of drawn copolyetheresters. The second factor determines the PBT plasticization effect of the soft blocks and a level of the interaction of chains in the amorphous mixed phase. Because of a weaker PBT plasticization effect of PTMO in comparison with that of the triblock, the deformation of the block copolymer E is accompanied by a more pronounced elevation of the glass transition temperature in comparison with the block copolymer P.     

In situ polarity functionalization and properties of styrenic triblock copolymers synthesized via RAFT emulsion polymerization

The in situ polarity functionalization of the styrenic triblock copolymers was accomplished via the block introduction of polar monomer, n‐butyl acrylate, with the help of reversible addition‐fragmentation chain transfer (RAFT) emulsion polymerization. The polarity functionalization, microphase separation, static and dynamic mechanical properties, water resistance, transparency, and thermal stability of the synthesized polarity‐functionalized triblock copolymers, polystyrene‐block‐poly(n‐butyl acrylate)‐block‐polystyrene (SAS), were extensively studied. The poly(n‐butyl acrylate) (PBA) middle block higher than 10 wt % has the favorable toughening effect on polystyrene (PSt) two‐end block due to the microphase separation in SAS. The glass transition of the continuous plastic phase (mainly composed of PSt block) has a much greater influence on the storage modulus than that of the dispersed rubber phase (mainly composed of PBA block). The polarity‐functionalized SAS has good water resistance, high transparency, and robust thermal stability. The polarity‐functionalized SAS will have such a potential application broadening as polar adhesive. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44603.     

Sulfonation and characterization of poly(styrene-isobutylene-styrene) triblock copolymers at high ion-exchange capacities

In this study, a triblock copolymer, poly(styrene-isobutylene-styrene), was sulfonated to eight different levels ranging from 0.36 to 2.04 mequiv./g (13 to 82 mol% of styrene; styrene is 19 mol% of the unsulfonated block copolymer). These sulfonated polymers were characterized with elemental analysis and infrared spectroscopy to confirm sulfonation and determine accurate sulfonation levels. Infrared analysis revealed four additional stretching vibrations as a result of sulfonation. Also, a linear relationship between absorbance at 1006 cm⁻¹ (stretching of the aromatic ring in styrene caused by the para-substituted sulfonic acid) and sulfonation level (measured by elemental analysis) was found. The density and water solubility of all the sulfonated polymers were measured and increased with increasing sulfonation level, as high as 1.31 g/cm³ and 351 wt%, respectively. In addition, a sulfonated triblock copolymer at 79 mol% sulfonation was neutralized with a cesium cation and revealed an increase in density, but a reduction in water solubility. This study demonstrates the resulting unique properties of sulfonated styrene-based block copolymers at higher ion-exchange capacities than previously reported.     

Functionalization of amphiphilic coil-rod-coil triblock copolymer poly(2-vinylpyridine)-b-poly(n-hexyl isocyanate)-b-poly(2-vinylpyridine) with florescence moiety and C60

We recently achieved quantitative synthesis of an amphiphilic coil-rod-coil triblock copolymer, poly(2-vinylpyridine)-b-poly(n-hexyl isocyanate)-b-poly(2-vinylpyridine), by coupling in situ living diblock copolymer poly(2-vinylpyridine)-b-poly(n-hexyl isocyanate) (P2VP-b-PHIC) using malonyl chloride in the presence of pyridine. This led to the introduction of an active methylene group that is a site for further functionalization in the rod block. The Michael addition reaction of the triblock copolymer with 7-(4-trifluoromethyl) coumarin acrylamide led to copolymer bearing a fluorescent pendent in the rod block. The fluorescent labeled copolymers were isolated in ∼94% yields. Similarly C₆₀ pendent was introduced to the rod block by the Bingel reaction. The yields of C₆₀ functionalized copolymers were ∼54%. The precursor and functionalized amphiphilic coil-rod-coil copolymer show diverse morphologies, such as micelles and vesicles by simply changing the solvent. For the C₆₀ functionalized block copolymer, structural constraints in micelles and vesicles prevented C₆₀ pendents to aggregate.     

Synthesis, morphology and mechanical properties of linear triblock copolymers based on poly(α-methylene-γ-butyrolactone)

A series of well-defined triblock copolymers containing middle soft poly(n-butyl acrylate) (PBA) block and outer hard blocks of poly(α-methylene-γ-butyrolactone) homopolymer (PMBL) or random poly (α-methylene-γ-butyrolactone)-r-poly(methyl methacrylate) copolymer (PMBL-r-PMMA) were synthesized by atom transfer radical polymerization (ATRP). Phase separated morphologies of cylindrical or spherical hard block domains arranged in the soft PBA matrix were observed by atomic force microscopy and small-angle X-ray scattering. The mechanical and thermal properties of the copolymers were thoroughly characterized and their thermoplastic elastomer behavior was studied. Dynamic mechanical analysis (DMA) showed for all PMBL-b-PBA-b-PMBL copolymers a very broad rubbery plateau range extending up to temperatures of 300 °C. Replacement of the PMBL hard block with the less brittle PMBL-r-PMMA resulted in an improvement of the tensile properties, without compromising the very good thermal stability of the materials.     

Micellization and phase transition behavior of thermosensitive poly(N-isopropylacrylamide)-poly(?-caprolactone)-poly(N-isopropylacrylamide) triblock copolymers

Thermosensitive triblock copolymers with two hydrophilic poly(N-isopropylacrylamide) blocks flanking a central hydrophobic poly(?-caprolactone) block were synthesized by atom transfer radical polymerization. Core-shell micellization of the triblock copolymers was inferred from the ¹H NMR spectra derived in two different solvent environments (CDCl₃ and D₂O). The micellar characteristics of these amphiphilic triblock copolymers were studied by pyrene fluorescence techniques, dynamic light scattering and transmission electron microscopy. The critical micelle concentrations of the triblock copolymers were in the range of 4-16 mg/L and the partition coefficients were in the range of 3.10 × 10⁴ to 2.46 × 10⁵. The mean diameters of the micelles, measured by light scattering, were between 90 and 120 nm. The temperature sensitivity of the triblock copolymers was demonstrated by the phase transition of a 250 mg/L aqueous polymer solution at the lower critical solution temperature (LCST). The enthalpy of the phase transition was determined by differential scanning calorimetry. PM3 quantum mechanical calculation method was used to understand the intermolecular interactions between the copolymer and the water molecules. A modular approach was used to simulate the phase transition observed at the LCST.     

Characterization and properties of amphiphilic block polymer based on poly(ethylene oxide) and poly(butyl acrylate)

PEO‐b‐PBA [PEO: poly(ethylene oxide); PBA: poly(butyl acrylate)], an amphiphilic block copolymer, prepared by redox radical polymerization, was characterized by infrared and ¹H nuclear magnetic resonance spectroscopy. The result revealed the existence of PEO and PBA segment in purified block copolymer. The thermal behavior of the block copolymer was determined by differential scanning calorimetry. With the introduction of PBA noncrystalline segment, the crystallinity of PEO was decreased. The emulsifying and water absorptive properties of PEO‐b‐PBA were also examined. It was found that the emulsifying volume and type were dependent on the amount of block copolymer and the PEO content in block copolymer. Under a certain range, the emulsifying volume increases with an increase of PEO content. The more the PEO content in block copolymer, the stronger the water absorptivity was. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 3432–3436, 2003     

Composites of polyamide 6 and silicate nanotubes of the mineral halloysite: Influence of molecular weight on thermal, mechanical and rheological properties

In this work, the potential of silicate nanotubes of the naturally occurring mineral halloysite as filler for polyamide 6 (PA 6) nanocomposites is evaluated. Several PA 6/halloysite composites with 0 wt% to 30 wt% filler loading using two different grades of PA 6 were prepared. In order to elucidate the influence of molecular weight on the properties of the nanocomposites, mechanical resp. rheological experiments (i) below the glass transition temperature T_g of PA 6, (ii) between T_g and the melting temperature T_m of PA 6 and (iii) above T_m were performed. Our investigations reveal that the addition of halloysite nanotubes favours the formation of the γ-modification for the low molar mass PA 6. Furthermore, the storage modulus, the tensile modulus and the yield stress of the composites increase with concentration of halloysite, an effect which is strongly pronounced at very low filler fractions for the low molar mass PA 6 composites. The increase of the storage modulus which was measured in dynamic-mechanical experiments is mostly dominant in the temperature interval from 55 °C to 100 °C, i.e. above the glass transition temperature of PA 6. Rheological investigations showed that the shear viscosity is only moderately increased by the addition of a low fraction of halloysite to PA 6, and nanocomposites with 30 wt% halloysite can be still processed. In summary, halloysite nanotubes are promising and inexpensive candidates for increasing the stiffness of PA 6 while maintaining very good flow properties.     

Exfoliation of organoclay nanocomposites based on polystyrene-block-polyisoprene-block-poly(2-vinylpyridine) copolymer: Solution blending versus melt blending

Exfoliated nanocomposites based on polystyrene-block-polyisoprene-block-poly(2-vinylpyridine) (SI2VP triblock) copolymer were prepared by solution blending and melt blending. Their dispersion characteristics were investigated using transmission electron microscopy, X-ray diffraction, and small-angle X-ray scattering (SAXS). For the study, SI2VP triblock copolymers with varying amounts of poly(2-vinylpyridine) (P2VP) block (3, 5, and 13 wt%) and different molecular weights were synthesized by sequential anionic polymerization. In the preparation of nanocomposites, four different commercial organoclays, treated with a surfactant having quaternary ammonium salt, were employed. It was found from SAXS that the microdomain structure of an SI2VP triblock copolymer having 13 wt% P2VP block (SI2VP-13) transformed from core-shell cylinders into lamellae when it was mixed with an organoclay. It was found further that the solution-blended nanocomposites based on a homogeneous SI2VP triblock copolymer having 5 wt% P2VP block (SI2VP-5) gave rise to an exfoliated morphology, irrespective of the differences in chemical structure of the surfactant residing at the surface of the organoclays, which is attributable to the presence of ion-dipole interactions between the positively charged N⁺ ion in the surfactant residing at the surface of the organoclay and the pyridine rings in the P2VP block of SI2VP-5 and SI2VP-13, respectively. Both solution- and melt-blended nanocomposites based on microphase-separated SI2VP-13 having an order-disorder transition temperature (T_ODT) of approximately 210 °C also gave rise to exfoliated morphology. However, melt-blended nanocomposite based on a high-molecular-weight SI2VP triblock copolymer having a very high T_ODT (estimated to be about 360 °C), which was much higher than the melt blending temperature employed (200 °C), gave rise to very poor dispersion of the aggregates of organoclay. It is concluded that the T_ODT of a block copolymer plays a significant role in determining the dispersion characteristics of organoclay nanocomposites prepared by melt blending.     

Phase transitions in mesophase macromolecules: The transitions of poly(p-acryloyloxybenzoic acid)

Poly(p-acryloyloxybenzoic acid) has been obtained in twelve single and two-phase physical states, which include the amorphous glassy and liquid states, the mesomorphic glassy and the mesomorphic liquid states and in addition eight two-phase semicrystalline states (crystal forms I and II). Using mainly differential scanning calorimetry, the transition temperatures, energies and heat capacity changes at the glass transitions have been studied. The time dependency of the glass transition has also been determined. The strongly heating rate dependent amorphous glass transition occurs at 348K (20k min^?1 heating rate, ΔC_p = 39 JK^?1 mol^?1), the mesophase glass transition, at 408K (also at 20k min^?1, ΔC_p = 43 JK^?1 mol^?1). The latter is less heating rate dependent. The amorphous to mesophase transition occurs between 375 and 475K (ΔH = ?4.5 KJ mol^?1); the peaks of melting transitions, which are also strongly heating rate dependent, were observed at 573K and 553K (20 K min^?1 heating rate) for crystal forms I and II, respectively. The heat of fusion of crystal form I is estimated to be 22 KJ mol^?1. There seems to be no partially amorphous-partially mesomorphic state.     

Synthesis of triblock copolymer having alternating structures and kinetics study on RAFT‐mediated bulk,miniemulsion and seed miniemulsion polymerizations

We have conducted reversible addition‐fragmentation chain transfer (RAFT) polymerizations of styrene (St) and maleic anhydride (MAh) and n‐butyl acrylate (BA) to produce a well‐defined triblock copolymer having alternating structure, P(St‐alt‐MAh)‐b‐PSt‐b‐PBA, via bulk, miniemulsion and seed miniemulsion polymerizations. The polymerization kinetics and living characters were investigated. The results followed by gel permeation chromatography (GPC) showed that bulk and miniemulsion polymerizations exhibited controlled nature such as narrow polydispersity index (PDI), controlled molecular weight, and first‐order polymerization kinetics, whereas triblock copolymer owned a rather wider PDI. Comparison of GPC RI and UV traces revealed that alternating copolymer and diblock copolymer have a very high percentage of living chains. For seed miniemulsion polymerization, when the molecular weight of triblock copolymer is more than 36,000 g/mol, the formation of homopolymer of BA resulted in broadening of PDI. ¹H NMR method was used to identify the compositions of block copolymers. Differential scanning calorimetry analysis showed that the copolymers exhibited distinct glass temperatures. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers