The shape memory performance of double crystalline poly(butylene succinate)-co-poly(ε-caprolactone) (PBS-co-PCL) multiblock copolymers with controlling microstructure was studied, and the corresponding microstructure origin was further quantitatively analyzed by wide and small-angle X-ray scattering (WAXS and SAXS) experiments. It was found that the multiblock copolymer with higher PCL content, proper deformation strain, and inhibited crystallization of PBS (lower crystallinity and smaller crystal size, which could be realized by quenching from the melt) would exhibit better shape memory fixity and recovery performance. WAXS and SAXS results revealed that the shape fixity ratio (Rf) was closely related with the relative crystallinity of the PCL component, while the shape recovery ratio (Rr) strongly relied on the deformation and recovery behavior of the PBS and PCL components that changed along with compositions and deformation strains. For the copolymer with higher PCL content (BS30CL70), at the lower deformation strain (0% ∼ 90%), both the PBS and PCL components after recovery had no orientation (labeled as stage I), resulting in almost complete recovery; with the deformation strain increasing (90% ∼ 200%), it was the irreversible deformation of the PCL component that largely took responsibility for the decreased Rr (stage II). On the contrary, when the PCL content decreased to 50 wt % (BS50CL50), stage I (0% ∼ 50%) and stage II (50% ∼ 100%) appeared in much lower strains; with the deformation strain increasing (100% ∼ 200%), the irreversible deformation of both PBS and PCL components was mainly responsible for the further reduction of Rr (stage III). It could exhibit excellent shape memory performance for biodegradable double crystalline multiblock copolymers by controlling the composition, deformation strain, and crystallization, which might have wide application prospects in biomedical areas.Keywords: composition and deformation strain; double crystalline multiblock copolymers; irreversible deformation; microstructure origin; shape memory fixity and recovery;

The classic deformation mechanism of the Gaussian model of Haward and Thackray was utilized to treat the true stress−strain behaviors of a family of novel polyether-b-amide segmented copolymers based on the crystalline hard segments of polyamide1012 and the amorphous soft segments of poly(tetramethylene oxide). The results showed that the deformation behaviors of the plastic copolymers abided by the Gaussian model, causing the modulus G of the strain-induced hardening process to depend on the weight percentage of the polyamide segments in the copolymers. By contrast, the deformation of elastomeric copolymers deviated from the model because of the occurrence of strain-induced crystallization in the soft polyether sections at large strains, which negated the Gaussian assumption; i.e., the random coil conformation was maintained even under substantial stretching. The onset point of deviation was, for the first time, quantitatively identified by in situ FTIR and further confirmed by in situ WAXD/SAXS.

Long chain polyamides with various number of methylene units in recurring amide groups, PA1012 and PA612, were blended to combine their unique advantages. The Brill transition and accompanied lamellar thickening were investigated by in situ wide angle X-ray scattering (WAXS) and small angle X-ray scattering. From WAXS patterns, the transformation from the α- to γ-crystalline phase, known as “Brill transition”, can be independently observed in the constituent phases of the long chain polyamide alloys (LCPAs) during heating. A constant Tb (ca., 100 °C) irrespective of the blend composition and proportional variations of the phase content was obtained. Additionally, with elevated temperature, a gradual increase in both the crystalline layer (Lc) and amorphous layer (La) was detected in constituent polyamides. The compositional independence of the Brill transition in LCPAs and similar lamellar thickening originate from the complete immiscibility of both polyamides, which share stronger intramolecular rather than intermolecular hydrogen-bonding interaction and hence exhibit self-association. Contributed by the γ phase, with less extended structure and increased lamellar thickness with compact stacking, LCPAs with controlled strength and flexible features can be achieved, which can be utilized in advanced mechanical applications, particularly for hoses of automobiles. The unusually linear compositional dependence of mechanical parameters makes it possible to tailor the polymorphic and tensile properties.Keywords: Brill transition; lamellar thickening; long chain polyamide blends; mechanical application; tensile performance;

•Mechanical property of PI fibers can be regulated by post annealing process.•Microvoid and liquid crystal-like structures determine primary mechanical property of PI fiber.•Annealing induced microvoid evolution enhances the PI fiber’s tensile modulus.•A simple method was proposed to improve PI fibers’ mechanical property.The correlation between mechanical property and the microstructure of two-step wet spinning polyimide (PI) fibers after thermal annealing was investigated. In-situ wide-angle X-ray diffraction (WAXD) and ex-situ small angle X-ray scattering (SAXS) methods were utilized to study the liquid crystal-like structure orientation and microvoids structure (the length and the orientation angle of microvoids) for PI fibers during annealing process. The effects of annealing temperature and time on the microstructure were systematically studied. The results reveal that the liquid crystal-like structure and the orientation degree keep nearly the same after annealing. Both of the molecular chain relaxation and the evolution of aligned microvoids are attributed to the change in tensile modulus. In low pre-drawing fiber, a slight decrease in microvoid length and an obvious rise in microvoid orientation angle lead to improved tensile modulus after annealing. In highly pre-drawn fiber, the tensile modulus decreases first due to de-orientation of polymer chains and then increase owing to disordering microvoids with decreasing size. Our work demonstrates the relationship between microvoids and mechanical properties in annealed PI fibers which provides a new avenue to access high performance PI fiber by facile annealing method and a deep insight into the structure and properties of high performance PI fibers.Download high-res image (118KB)Download full-size image

•The self-associated blends, long chain polyamide alloys (LCPAs), are researched.•The immiscible LCPAs show mechanical compatibility with strong interfacial adhesion.•The concurrence of fracture reinforcement and toughness is achieved above Tg.•Strain-induced crystallization and microfibrillar structure occur upon stretching.•The correlation between microstructure and mechanical performance is established.The deformation-induced microstructure evolution of self-associated polymer blends, i.e., long chain polyamide alloys (LCPAs), was investigated and correlated with their mechanical performance. A PA1012 (soft phase)/PA612 (hard phase) blend was selected for this study. It is interesting that both Young's modulus and yield stress exhibit a nearly linear dependence with composition, which follows a simple additive mixing law. With strong intra-associated hydrogen bonds, the LCPAs studied here were found to be immiscible, but mechanically compatible because of the strong interfacial adhesion of the two constituents. Moreover, when the blend was deformed close to or above its glass transition temperature (Tg), the simultaneous occurrence of fracture reinforcement and toughness was achieved, which was defined as “soft phase-reinforcing-hard phase” (SRH). On the other hand, below Tg, a “hard phase-reinforcing-soft phase” (HRS) mechanical behavior was identified at the expense of elongation at break. Upon uniaxial deformation above Tg, the PA1012 component imparted to the blends a superior fracture stress due to strong strain-induced crystallization effect and the subsequent formation of a microfibrillar structure. However, the PA612 phase contributed little to the improvement of the mechanical properties of the blends. The large discrepancy in the contributions of the blend components to the fracture stress, primarily originates from the flexible nature of the PA1012 chains and the thin crystals nucleated by the already crystallized PA612 phase. However, below Tg, tilt, slippage and fragmentation of the lamellae occur in both phases, which are accompanied by apparent void formation and directly lead to catastrophic fracture. Considering the higher density of hydrogen bonds in PA612, the PA612-rich blends displayed higher fracture stresses as compared to the rest of the compositions. Overall, the distinctive microstructure evolution of the constituent phases and their roles in stress enhancement at large strains have been successfully established. The results presented here shed light on the deformation mechanism of self-associated polymers and offer a new pathway for the development of mechanically reinforcing materials.Download high-res image (244KB)Download full-size image

•Elastic filament fiber is developed by long chain poly(amide-block-ether) (LPAE).•The hard segment of the copolymer is long chain polyamide (LCPA).•LCPA hard segments are synthesized from bio-fermenting monomers.•The fiber exhibits low initial modulus, high elongation and excellent elasticity.•Good elasticity and reversibility is from alternative distribution of hard/soft domains.A new kind of elastic filament fibers based on long chain poly(amide-block-ether) (LPAE) was successfully prepared by simple melt spinning. The LPAE elastomer was synthesized with hard blocks of long chain polyamide (LCPA) oligomers, produced by bio-fermenting monomers, and poly(tetra-methylene ether) glycol (PTMEG) soft blocks. The mechanical and thermal results demonstrated that it represented large elongation at break, low initial modulus and excellent elastic recovery, which were comparable to that of commercially used spandex within strain of 200% and superior to that of olefin-based XLA fibers. The high elasticity and reversibility arise from that the LCPA hard segments, representing semi-crystalline state at ambient temperature and generating three dimensional hydrogen bonds between adjacent chains, serve as the physical crossing linking sites, while the polyether soft segments can be deformed largely due to amorphous state and the easy conformation changes of polymer chains. However, beyond 200%, the recovery gradually drops and it origins from the strain-induced crystallization and slight crystal transition in soft and hard segments, respectively. Hence, the novel elastic filament fibers retain the merits of the segmented copolymer and LCPA like better dimensional stability, chemical and abrasion resistance, and better soft handle. With this research, LPAE elastic fiber, featuring excellent elasticity and reversibility, is confirmed to be a promising candidate to replace spandex and XLA fibers in some typical textile applications.Download high-res image (208KB)Download full-size image

•We synthesized a family of novel polyether-b-amide segmented copolymers based on polyamide1012 and PTMEG.•The Brill transitions of polyamide component in homo polymer and copolymers were investigated by in situ WAXD/FTIR/SAXS.•The Brill temperatures of homo PA1012 and copolymers fall into a narrow range from 115 °C to 125 °C.•The lamellar thickness of homo PA1012 and copolymers is found to close to each other.In present work, the Brill transitions of polyamide component in polyether-b-amide (PEBA) segmented copolymers were investigated by a combination of in situ wide-angle X-ray diffraction (WAXD), Fourier transform infrared spectroscopy (FTIR) and small-angle X-ray scattering (SAXS). The lab-made, additive-free PEBA copolymers, which consist of crystalline polyamide1012 (PA1012) hard segments (HS) and amorphous poly(tetramethylene oxide) (PTMO) soft segments (SS), provide an opportunity to explore the relationship between the Brill temperature (TB) and the composition of PEBA, as well as the impact of incorporation of PTMO on the variation of the Brill transition. It was found that the TB of PA1012 component kept approximately constant during the cooling process and the second heating process for homo PA1012 and HS-dominant PA1012-PTMO copolymers. Manifested itself by the merging of two diffractions of (100) and (110)/(010), the Brill transition for each sample during the second heating occurred with a TB varying from 115 °C to 125 °C. This temperature range can be referred as a TB zone. The FTIR band at 1188 cm−1 disappeared and FTIR bands sensitive to crystalline phase blue shifted from 939 cm−1 to 943 cm−1, when the temperature reached the respective TB for each sample. Based on the calculation of one-dimensional electron density correlation function, lamellae thickness in the initial states of all PA1012 and PA1012-based PEBA samples was found to be independent on the composition, which resulted in the consistent TB.Download high-res image (55KB)Download full-size image

PA1012/calcium silicate whisker nanocomposites with contents of whisker ranging from 10 wt% to 40 wt%, are prepared by twin screw extruder without any additions of coupling agent. The effect of whisker on the matrix is analyzed by the studies of morphology, the mechanical properties, water absorption and thermal stabilities. SEM micrographs obviously demonstrate, even under the high filler content of 40 wt% and without surface treatment, calcium silicate whisker can be homogeneously dispersed in polyamide, directly leading to the enhanced mechanical properties. The mechanism of higher efficiency of reinforcement is needle-like shape whisker, having access to be intercalated, and mutual affinity caused by hydrogen bonding interaction between carbonyl group in polyamide chain and hydroxyl group in whisker surface. Both aspects attach matrix with excellent stress-transfer properties. In addition, with the assistance of whisker, the nanocomposite favors an improved water absorption as well as thermal stability, which is intimately associated with physical performance.

•Facile preparation of a new triple-shape memory polymer material, PCL/PVC blend, was achieved.•The PCL-30% had a broad glass transition and displayed the mechanical properties like an elastomer.•The nano-scale PCL crystals and chain entanglements served as the physical networks.•The amorphous PVC and PCL regions acted as the reversible phase at high and low switch temperatures respectively.Poly(ε-caprolactone) (PCL) and poly (vinyl chloride) (PVC) are miscible with each other, and their glass transition temperatures differ a lot. With this feature, we tried to design a triple-shape memory material by simply blending them together. After carefully examining the glass transition broadness and the tensile behavior of various PCL/PVC blends, PCL-30% (the weight fraction of PCL was 30%) was picked out as the best candidate. It fulfills two criterions that it has a wide glass transition range and it has no strain or stress induced crystallization. Subsequent triple-shape memory procedure proves that it could function as an effective shape memory blend. The structure was studied by SAXS/WAXS and nano-scale periodic structure was found, which was composed of crystalline and amorphous PCL regions of 4.7 nm and 8.4 nm respectively. Such periodic domains would orient along the stretch direction, but it kept stable and could restore to its original state after the stress was released. Dichromic FTIR discerned the orientation of PCL chains at low temperature and PVC chains at high temperature. It can be concluded that both the nano-scale PCL crystalline structures and physical entanglements served as the fixed phase, while the amorphous PVC and PCL regions played as the reversible phase at high and low switch temperatures respectively.

•Strain-induced microstructure changes of long chain polyamide are studied above Tg.•The transient microstructure is revealed at intermediate strain.•It’s from the approach of tilted lamella, the insertion and orientation of new one.•The temperature dependence shows the role of new lamellae in strain-hardening.•The correlation between microstructure and mechanical response has been made.Deformation-induced microstructure evolution of Polyamide 1012, with an emphasis on lamella development, polymorphism transition and molecular orientation, is investigated. When deformed above Tg (100 °C), a series of complex SAXS patterns are continuously identified at intermediate strain including four-point pattern, figure-8 pattern and X-shaped pattern, accompanied with two-bar pattern on the meridian, which corresponds to the transient microstructure. Such particular structure is resulted from the approach of tilted lamellae along the drawing direction, the insertion and the orientation of new lamellae. To investigate the temperature dependence, the microstructure developments at 30 °C (below Tg), 60 °C (close to Tg) and 100 °C (above Tg) are compared. Considering 38% enhancement of the fracture stress with elevated temperature (from 60 °C to 100 °C), the temperature dependence of particular SAXS patterns further suggests that the elongation above Tg favors higher level of stretching-induced crystallization and better aligned microfibrillar structure, which competes with thermal-induced increased chain mobility. Based on the comprehensive results, the correlation between microstructure and mechanical response has been successfully established, which is featured by the synchronous occurrence of transient structure with slight strain hardening.

In this work, the superior shape memory properties and microstructure evolution of poly(ether-b-amide12) (PEBA) enhanced by poly(ε-caprolactone) (PCL) were investigated. The PEBA/PCL blends were prepared by the methods of solution mixing and compression molding. The representative phase-separated morphologies were observed by optical microscopy (OM). The recovery stress and shape memory properties of the samples were determined by dynamic mechanical analysis (DMA). It was found that high loadings of PCL (up to 35 wt%) only marginally worsened the recovery properties of the blends. However, the values of maximum recovery stresses increased greatly with PCL content in spite of deformation temperature, indicating an enhancement effect of PCL on the recovery stress of polymer blends. Moreover, in situ wide angle X-ray scattering (WAXS) investigation revealed the microstructure evolution during the shape memory process. It was notable that during the stretching process, the strain-induced crystallization of soft segments (PTMO) of PEBA shifted to lower strain due to the existence of the PCL phase. The present results provided a reference that the recovery stress could be greatly increased by adding another polymer to the shape memory polymer, which is vital to application in the biomedical field.

•Flow-induced crystallization of long-chain aliphatic polyamides (LCPA) was studied.•The extensional-shear-coupled flow field was achieved by a micro-injection moulding device.•The crystallization morphology of LCPA under complex flow field was well preserved.•A novel inverted anisotropic crystallization structure was characterized by μWAXS and SAXS.•Stretched LCPA chains by extensional pulse at different shear regime relaxed in different ways.The present work deals with the flow-induced multiple orientations and crystallization structure of polymer melts under a complex flow field. This complex flow field is characteristic of the consistent coupling of extensional “pulse” and closely followed shear flow in a narrow channel. Utilizing an ingenious combination of an advanced micro-injection device and long chain aliphatic polyamides (LCPA), the flow-induced crystallization morphology was well preserved for ex-situ synchrotron micro-focused wide angle X-ray scattering (μWAXS) as well as small angle X-ray scattering (SAXS). An inverted anisotropic crystallization structure was observed in two directions: perpendicular and parallel to the flow direction (FD). The novel anisotropic morphology implies the occurrence of wall slip and “global” fountain flow under the complex flow field. The mechanism of structure formation is elucidated in detail. The experimental results clearly indicate that the effect of extensional pulse on the polymer melt is restrained and further diminished due to either the transverse tumble of fountain flow or the rapid retraction of stretched high molecular weight tails. However, the residual shish-kebab structures in the core layer of the far-end of channel suggest that the effect of extensional pulse should be considered in the small-scaled geometries or under the high strain rate condition.

Polyester-based biodegradable polyurethane (PU) with different hard segment ratios was selected to modify the impact toughness of poly(L-lactide) (PLLA). The influence of blending composition and hard segment ratio of PU on the phase morphology, crystallization behavior and mechanical properties of PLLA/PU blends has been investigated systematically. The results showed that the PU particles were uniformly dispersed in PLLA matrix at a scale from submicrons to several microns. The glass transition temperature of PU within these blends decreased compared to that of neat PU, but rose slightly with its content and hard segment ratio. The presence of PU retarded the crystallization ability of PLLA, whereas enhanced its elongation at break and impact resistance effectively. As the PU content reaches up to 30 wt%, the phenomenon of brittle-ductile transition occurred, resulting in a rougher fracture surface with the formation of fibril-like structure. Moreover, under the same concentrations, the elongation at break and impact strength of PLLA blends decreased slightly with the increase of hard segment ratio of PU.

A low molecular weight aliphatic amide, N, N′-ethylenebis (12-hydroxystearamide) (EBH), was selected to tailor the crystallization behavior of poly (l-lactide) (PLLA). The effect of EBH on the crystallization kinetics, fine crystalline structure, and molecular mobility of PLLA has been systematically investigated. It has been found that the crystallizability of PLLA, including crystallization rate and crystallinity, can be promoted significantly by the addition of only 1 wt% EBH. Both the nucleation density and linear growth rate of spherulites have been improved, which together contributed to the decrease of overall crystallization time. The long period and lamellar thickness of PLLA crystals increased gradually with the isothermal crystallization temperature, whereas they were less influenced by the incorporation of EBH. Dynamic mechanical analysis proved that the mobility of PLLA chains was also increased in the presence of EBH. The accelerating effect of EBH on both the nucleation and molecular mobility of PLLA was supposed to be the hydrogen-bonding interaction between the hydroxyl groups in EBH and carboxyl groups in PLLA.

•Transamidation (TA) determination and miscibility in PA1012/612 are discussed.•Factors affecting TA includes annealing temperature, time, component and category.•The total degree of enthalpy decrease ratio ψ indicates the extent of reaction.•ψ is mainly dominated by several factors through which TA degree can be controlled.•Interface of polyamides is improved efficiently due to copolyamide locally formed.•Tailored LCPAs can be established by regulating the transamidation extent.Transamidation between PA1012 and PA612 was investigated with combination techniques including differential scanning calorimetry (DSC), rheometry, nuclear magnetic resonance (NMR) and variable-temperature Fourier transform infrared spectroscopy (VT-FTIR). Based on the increase of storage modulus with sweep time, the presence of reactive chain ends were proved, promoting chain growth in either polyamide and interchange reaction in binary blend at high temperature. NMR and VT-FTIR testing signals sufficiently convinced the expected interchange reaction. Quantitative data analysis of DSC and NMR provided direct evidences for evaluating the exchange degree. Various experimental parameters were systematically considered, including reaction temperature and time, blending composition and categories of polyamides. Raising the reaction temperature or prolonging the reaction time would accelerate the transamidation rate and degree. The interfacial miscibility between two LCPA components could be improved by the formation of copolyamides. This work provides a quantitative evaluating method to detect the transamidation extent between two aliphatic LCPAs that overcomes the traditional characterization limitation which is only feasible for whose polyamide component chain is with large volume structure like benzene ring.

•Effects of filler geometry on domain evolution during and after shear were studied.•OMMT (plate) filler resulted in much thinner domain structure under low shear.•SiO2 (globe) filler resulted in smaller and shorter domains under low shear.•Two fillers had no influence on the formation of string phase under high shear.•The recovery of OMMT sheets strongly influenced the relaxation process of domains.The influence of nanoparticle geometry, such as size and shape, on the phase morphology of partially miscible binary polymer blends under and after shear has been examined by rheological and rheo-optical techniques. The phase morphologies of the solution-polymerized styrene-butadiene rubber and low vinyl content polyisoprene (SSBR/LPI) blend systems were affected by the dispersion status of fillers which were determined by filler shapes and shear strength. Under weak shear flow, the domain morphology of the OMMT filled blend was much thinner than that of the SiO2 filled blend. Under strong shear flow, the string-like phase interface of the OMMT filled blend was much blurred compared with that of the SiO2 filled blend. After shear cessation, the orientation status of OMMT sheets determined the orientation of newborn domains. Combined morphology observation and rheological analysis showed that the anisotropic structure and the unfavorable bending energy of OMMT sheets played important roles on phase morphology and its evolution process during or after shear.

In this work, electrically and thermally actuated triple shape memory polymers (SMPs) of chemically cross-linked polycyclooctene (PCO)–multiwalled carbon nanotube (MWCNT)/polyethylene (PE) nanocomposites with co-continuous structure and selective distribution of fillers in PCO phase are prepared. We systematically studied not only the microstructure including morphology and fillers’ selective distribution in one phase of the PCO/PE blends, but also the macroscopic properties including thermal, mechanical, and electrical properties. The co-continuous window of the immiscible PCO/PE blends is found to be the volume fraction of PCO (vPCO) of ca. 40–70 vol %. The selective distribution of fillers in one phase of co-continuous blends is obtained by a masterbatch technique. The prepared triple SMP materials show pronounced triple shape memory effects (SMEs) on the dynamic mechanical thermal analysis (DMTA) and the visual observation by both thermal and electric actuations. Such polyolefin samples with well-defined microstructure, electrical actuation, and triple SMEs might have potential applications as, for example, multiple autochoke elements for engines, self-adjusting orthodontic wires, and ophthalmic devices.Keywords: co-continuous structure; electrical actuation; nanocomposites; polyolefins; selective distribution; triple shape memory effects

In this work, two-way shape memory (TWSM) properties and the corresponding structural origin of cross-linked poly(ε-caprolactone) (cPCL) with different gel contents obtained by adopting different weight percentages of benzoyl peroxide (BPO) were investigated. The effects of gel contents on melting temperatures, crystallization temperatures and crystallinity of the cPCL materials were studied with differential scanning calorimetry. The TWSM properties of the samples were determined by dynamic mechanical analysis. It was found that gel content was the key factor determining the TWSM behavior, and higher gel content would result in the so-called robust TWSM effect for cPCL samples (cPCLx, x denoting the weight percentage of BPO). Compared with cPCL10, cPCL5 had larger elongation and lower recovery capabilities due to its lower gel content. However, the sample with much lower gel content (cPCL3) displayed almost no TWSM behavior, implying that an appropriate gel content was responsible for the TWSM characteristic. The crystalline structure of cPCLx subsequently changed when subjected to external stress. As the samples were cooled down under constant stress, higher stress would lead to a more oriented crystalline structure. Furthermore, concurrent wide-angle and small-angle X-ray scattering investigations revealed the structural evolution occurring during the TWSM process, indicating that the crystal orientation along the stretching direction took place simultaneously with the elongation during the cooling process.

The rheological properties of polybutadiene (PB)/polyisoprene (PI) in the unstable and metastable regions under oscillatory shear flow has been studied. Based on the shear quench experiment, it showed that the time dependent storage modulus G′G′ initially increased and then decreased with time in the unstable region. The concentration fluctuation led to the increasing part and the coarsening of the bicontinuous structure led to the decreasing of the storage modulus. The rheological response of the nucleation/growth in the metastable region was similar to that of spinodal decomposition. The fluctuation-assisted nucleation theory proposed by Balsara et al. was similar to the spinodal decomposition mechanism, which led to the similar rheological response. However, the continuous nucleation led to the slow growth and the almost linear decayed rheological curve, which was different from that in the unstable region. According to this theory, the relative nucleation rate on the early stage of nucleation could be calculated, which increased exponentially with the reciprocal of quench temperature in the metastable region. The spinodal point under oscillatory shear was also determined by the difference of maximum value of the storage modulus and the bulk modulus.

•Biomedical PU porous membrane was prepared by water micro-droplet induced phase inversion.•The membrane was with uniform pores structure, good air permeability and mechanical properties.•Uniform pores structure of the membrane was good to improve mechanical properties.•The adjacent pores connected by a micro-hole and that resulted better air permeability.•The porous membrane formation mechanism based on the phase separation kinetics was proposed.Biomedical polyurethane (BPU) porous membranes with controlled morphology and excellent permeability and mechanical properties were prepared via a method involving a phase inversion induced by water micro-droplets, which were generated by an ultrasonic atomizer. The cross-section morphology, air permeability and mechanical properties of the porous membranes were investigated. The SEM images demonstrated that the adjacent pores were connected by a micro-hole, serving as a “backdoor” for the pore. An interconnected porous structure was obtained, improving the air permeability of the BPU membrane relative to the membrane produced by immersion precipitation. Our studies indicated that the diameter of the pores in the membrane depended on the solution viscosity, allowing porous membranes with a desired morphology to be obtained by adjusting the polymer concentration and solution viscosity. The application of micro-droplets of water during membrane preparation reduced the exchange rate between the solvent and nonsolvent, resulting in the microphase separation of polymer molecules and the formation of a uniform porous structure in the membrane, which improved the air permeability and mechanical properties of the BPU porous membranes. This is a simple and effective preparation method for high-performance porous membranes with potential applications in tissue engineering scaffolds, controlled-release drug delivery and vascular grafts.

The morphology evolution and the corresponding linear viscoelastic behavior of the phase-separating polybutadiene (PB)/low vinyl content polyisoprene (LPI) blend have been investigated by phase contrast optical microscopy (PCOM), small-angle light scattering (SALS) and rheometry. Two kinds of structure evolutions and rheological responses have been observed. It is found that the co-continuous structure generally gives a power law behavior of the dynamic storage modulus versus frequency and the coarsening of co-continuous structure leads to a decrease of the storage modulus. For the droplet-matrix structure, a platform modulus is observed at the mediate frequencies, followed by the typical terminal relaxation behavior of storage modulus at the extremely low frequencies. The decreasing platform modulus and increasing terminal modulus with the growth of droplets are observed and can be well interpreted by the simplified Palierne model. The platform modulus and terminal modulus at a given frequency are found to be scalable with the phase separation time. Besides, the characteristic relaxation time and domain size of the droplets have been obtained by rheology. And it seems that the rheologically determined droplet dimensions are consistent with the ones determined by PCOM and SALS.

The structure, porosity and crystallization behavior of poly (L-lactic acid) and poly (L-lactic acid)/polyurethane porous membranes, prepared from ethanol/dioxane and ethanol/water coagulation baths through immersion precipitation, have been systematically investigated. The diffusion rate between solvent and nonsolvent as well as the equilibrium phase diagram of PLLA/solvent/nonsolvent system were also well studied. It has been proved that the ultimate structure and performance of the membranes could be mediated under control by suitable adjustment on phase separation behavior of the ternary system through varying coagulation bath compositions. The results show that the presence of lower ratio of dioxane in ethanol baths endows the resulting membranes with uniform sponge-like structure, higher porosity and crystallinity due to the moderate solidification and crystallization of PLLA, while increasing the water concentration tends to have a modestly opposite effect and obtains membranes with irregular finger-like structure, lower porosity and crystallinity. Under the same coagulation baths, PLLA/PU membranes possess slightly larger pores size and porosity than pure PLLA membranes, but the presence of PU appears to have no effect on the crystallinity of PLLA.

The growth of transcrystallites induced by fracture stress was studied in the solution casted polystyrene (PS)/poly(ethylene oxide) (PEO) blends films. Both the structure geometry of the crack edge and the effect of local fracture stress on the crystallization were considered. It was found that the local fracture stress contribution is the most important factor to the formation of transcrystallites. Transcrystallites were obtained when PEO crystallized before the local fracture stress was relaxed, while only regular spherulites were obtained when crystallization took place after the relaxation of the local fracture stress.

The morphological and rheological responses to the transient and steady shear flow for a phase-separated polybutadiene (PB)/low vinyl content polyisoprene (LPI) blend have been investigated. Under steady shear flow where the applied shear rate is not too large, the steady sheared structures become increasingly anisotropic and interconnected with an “en route” to the formation of string phases as shear rate increases. After that, the further increase of shear rate leads to a blurred domain interface. These shear-induced complex structures in turn affect the rheological response greatly and both the shear thinning and shear thickening were observed in the steady shear behavior of the phase-separated PB/LPI blend. Under transient shear flow, the time (or strain) dependence of viscosity and morphology after a shear rate jump were extensively studied in order to obtain the insight into the steady state formation and found to be mainly determined by the final shear rates. Depending on whether the transient string phases which were formed by the transient shear flow can be stabilized and with clear domain interface, three kinds of transient shear viscosity changes have been observed. Some of the observations are quite different from the model immiscible blend and believed to be closely related to the significant shear-induced mixing effect happened in the PB/LPI blend.Graphical abstract

Isotactic polypropylene (iPP)/organo-montmorillonite (OMMT) nanocomposite was modified by poly(ethylene-co-octene) (PEOc). PEOc-rich domains were well dispersed in the iPP matrix, with narrowly distributed size. OMMT layers were well dispersed, mainly intercalated and partially exfoliated. Compared with the case in the binary composites, many OMMT layers were preferential distributed inside and around the PEOc-rich domains in the ternary composite samples, which formed an enhanced OMMT filler network. The reason for the OMMT preferential distribution was considered to be dragged or wrapped by PEOc-rich domains during sample preparation and phase separation.

The rheology of near- and off-critical elastomeric blends of polybutadiene (PB)/ low vinyl content polyisoprene (LPI) has been studied as a function of temperature, heating rate, and shear frequency. Depending on the composition, near or far away from the critical point, blends showed different behaviors in several aspects: temperature ramp curves, shift of the apparent binodal and spinodal points under oscillatory shear at the given frequency and strain amplitude. The composition dependent rheological responses are interpreted by the differences in the amplitude of the critical fluctuation near the phase boundary and in the shear induced mixing mechanisms between near- and off-critical blends. For near-critical blends, the critical fluctuation is large enough to induce a considerable extra stress when the blend is still in the miscible state. As a result, a heating rate independent upturn of G′ can be observed and the apparent spinodal point can be greatly shifted through the strong suppression of the large critical fluctuations. In contrast, for off-critical blends, the critical fluctuations in the metastable region are relatively small and there is competition between the phase separation kinetics and the heating rate. Therefore, the blend displayed a heating rate dependent apparent binodal point and a large shift of the apparent binodal point but a moderate shift of the spinodal point under oscillatory shear. By lowering or extrapolating the measured frequency to a very small value (0.1 rad/s in this study) for both binodal and spinodal points, the rheologically determined phase diagram is consistent with the static results obtained by optical microscopy observations.

The phase diagram of an isotactic polypropylene/poly(ethylene-octene) copolymer (iPP/PEOc) blend system was investigated using phase contrast optical microscopy, laser light scattering and differential scanning calorimetry (DSC). The sample goes through immiscible (opaque) region to transparent region (seemingly miscible) and back to immiscible (opaque) again as temperature increases through 300 °C region. But it turns out that this is not a real one phase region. It is caused by a temperature dependent inversion of refractive indices between the two component polymers, which can be easily misinterpreted as a miscible region between an upper critical solution temperature (UCST) state and a lower critical solution temperature (LCST) state. With a proper interpretation and analysis of this refractive index inversion, the UCST phase diagram of this iPP/PEOc blend system has been obtained.

In this article, the rheological properties of polypropylene (PP)/ethylene–propylene–diene terpolymer (EPDM)/silicon dioxide (SiO2) ternary composites were systematically investigated. Two kinds of nano-SiO2 particles (with hydrophobic (denoted as A-SiO2) or hydrophilic (denoted as B-SiO2)) as well as two processing methods (one-step or two-step) were first employed to prepare PP/EPDM/SiO2 ternary composites. Then the deep mixing and morphology evolution of polymer composite with mixing time were assessed by rheological method, on the focus of formation of filler-network, and compared with scanning electron microscopy (SEM) observations. Linear viscoelastic behavior was observed for PP/EPDM and PP/SiO2 binary system, showing no evidence of the formation of filler-network structure. However, a solid-like rheological behavior, which was attributed to the formation of the filler-network structure as confirmed by SEM observation, could be observed in some PP/EPDM/SiO2 ternary systems, depending on the SiO2 surface property, processing method and EPDM content. It seemed that SiO2 with hydrophilic surface was necessary for the formation of filler-network in PP/EPDM/SiO2 ternary system. Besides, two-step processing method made the solid-like behavior occurred at an earlier stage compared with that of a one-step processing method, also, the higher elastomer content facilitated the formation of the filler-network structure. The results were in good agreement with those reported in our previous publications [Yang H, Zhang Q, Guo M, Wang C, Du R, Fu Q. Polymer 2006;47:2106] [Yang H, Zhang X, Qu C, Li B, Zhang L, Zhang Q, et al. Polymer 2007;48:860].

Viscoelastic polymer blends of polybutadiene (PB)/low vinyl content polyisoprene (LPI), with a lower critical solution temperature (LCST), show interesting rheological behaviors in temperature ramp measurements. In this report, a systematic study has been carried out, and the underlying physics has been investigated for the storage modulus G′ at various temperatures and shear frequencies as the system passes through the binodal and the spinodal phase boundary lines. We considered the nucleation mechanism, spinodal fluctuations, shear induced mixing, and rheological models in the interpretation of these interesting phenomena. Shear induced mixing is varied in our system, and the frequency dependence is obvious. Competition between the kinetics of the nucleation process and the droplet growth process has a prominent effect on the storage modulus for samples of noncritical compositions, while for samples with near-critical compositions the morphological evolution is responsible for the viscoelastic changes. Time-dependent experiments provide important information about morphological evolution at different temperatures. The region where fluctuations play a dominant effect on G′ can be discerned from our treatment of putting G′ and {G′(ω)/[G′′2(ω)T]}2/3 in the same reference frame. On the basis of the results from both heating and cooling processes, it seems that there also exist competition between fluctuations and interfacial gradient on the determination of the value of G′.