Crystallization Of Hard Segments Research Articles

Polylactide-Based Nonisocyanate Polyurethanes: Preparation, Properties Evaluation and Structure Analysis.

This study investigated the successful synthesis and characterization of nonisocyanate polyurethanes (NIPUs) based on polylactide. The NIPUs were synthesized by a condensation reaction of oligomers with hard segments (HSs) and synthesized carbamate-modified polylactic acid containing flexible segments (FSs). The oligomers with HSs were prepared from phenolsulfonic acid (PSA) or a mixture of PSA and hydroxynaphthalenesulfonic acid (HNSA), urea and formaldehyde. The mixing of oligomeric compounds with different amounts of formaldehyde was carried out at room temperature. Obtained NIPU samples with different hard segment content were tested for their mechanical and thermal properties. The tensile strength (TS) of all NIPU samples increased with an increasing amount of HSs, attaining the maximum value at an HS:FS ratio of 1:3. Samples prepared from PSA and HNSA showed higher tensile strength (TS) without significant change in elongation at break compared to the samples based only on PSA. Thermogravimetric analysis data indicated an absence of weight loss for all samples below 100 °C, which can be considered a safe temperature for using NIPU materials. Maximum degradation temperatures reached up to 385 °C. Fourier transform infrared spectroscopy results confirmed the existence of expected specific groups as well as the chemical structure of the prepared polyurethanes. DSC analysis showed the existence of two characteristic phase transitions attributed to the melting and crystallization of hard segments in the NIPU samples.

A Non-Isocyanate Route to Poly(Ether Urethane): Synthesis and Effect of Chemical Structures of Hard Segment.

A series of non-isocyanate poly(ether urethane) (PEU) were prepared by an environmentally friendly route based on dimethyl carbonate, diols and a polyether. The effect of the chemical structure of polyurethane hard segments on the properties of this kind of PEU was systematically investigated in this work. Polyurethane hard segments with different structures were first prepared from hexamethylene di-carbamate (BHC) and different diols (butanediol, hexanediol, octanediol and decanediol). Subsequently, a series of non-isocyanate PEU were obtained by polycondensation of the polyurethane hard segments with the polyether soft segments (PTMG2000). The PEU were characterized by GPC, FT-IR, 1H NMR, DSC, WAXD, SAXS, AFM and tensile testing. The results show that the urea groups generated by the side reaction affect the degree of crystallization of hard segments by influencing the hydrogen bonding of the hard segments molecular chains. The degree of hard segment crystallization, in turn, affects the thermal and mechanical properties of the polymer. The urea group content is related to the carbon chain length of the diol used for the synthesis of hard segments. When butanediol is applied to synthesize hard segment, the hard segment of the resulting PEU is unable to crystallize. Therefore, the tensile strength and modulus of elasticity of butanediol-based PEU is lowest among three, though it possesses the highest urea group content. When longer octanediol or decanediol is applied to synthesize the hard segment, the hard segments in the resulting polyether-based polyurethane are crystallizable and the resulting PEU possesses higher tensile strength.

Supercritical CO2 foaming and shrinkage resistance of thermoplastic polyurethane/modified magnesium borate whisker composite

Thermoplastic polyurethane (TPU) foam is eco-friendly prepared with supercritical carbon dioxide (CO2) as a blowing agent. Magnesium borate whisker (Mg2B2O5) modified by borate ester coupling agent (BE) (BE-Mg2B2O5) was introduced into TPU to modulate cell nucleation, bubble growth and foam stabilization depending on the intercalation structure of Mg2B2O5, entanglement effect of BE and increased physical crosslinking density. The modulation of Mg2B2O5 on the hard segment crystallization promoted hard segment’s (HS) microstructure, i.e., physical crosslinking density. The addition of BE-Mg2B2O5 enhanced the rheological properties, maximum tensile strength and elongation at break of TPU composites. Excellent mechanical property and strain-hardening behavior of TPU composites improved the foamability of TPU composites. Mg2B2O5 and entanglement points acted as heterogeneous nucleation sites to momentously increase cell density and reduce bubble size. Intercalation structure of Mg2B2O5, entanglement effect of BE and high physical crosslinking density effectively limited the TPU relaxation and memory recovery. BE-Mg2B2O5 around the cell wall acted as a barrier to slow down the diffusion rate of CO2 from inside the bubble to outside. The dual action synergistically reduced the shrinkage of TPU foam.

Effect of thermal history on the structure and mechanical properties of a thermoplastic polyester elastomer

An appropriate processing protocol was proposed to improve the rubber elasticity of a thermoplastic polyester elastomer (TPEE). Rheology measurements at the processing temperature and thermal measurements determined the critical temperature at which small numbers of crystals exist. It was found from dynamic mechanical analyses, wide- and small-angle X-ray measurements, and electron microscope observations that cooling from the critical temperature markedly promoted the crystallization of hard segments. Moreover, tensile tests revealed that the modulus and yield stress increased. Most importantly, compared with films obtained by cooling from high temperatures, the residual strain at a fixed stress was low—i.e., the rubber elasticity was improved owing to its well-developed crystalline structure. These effects were accelerated by a more optimized process—i.e., subsequent heating at slightly below the critical temperature. The technique is noteworthy because a TPEE with high modulus generally shows poor elasticity.

Polyurethane-urea-amide-based triblock copolymer contains functionalized polystyrene: Synthesis, characterization and solvent resistivity study

Synthesized polystyrene (PSt) with a molecular weight of 2100 g/mol, hexamethylene diisocyanate (HMDI), 6-hexylamino benzamide (6B), and N1-(6-aminohexyl)-N4-(6-benzoylaminohexyl)terephthalamide (6T6B) are used to make a copolymer of thermoplastic elastomers. The prepared polymers’ inherent viscosity results (0.4–1.1 dL/g) support the polymer’s high mass. The presence of a monodentate urea group in the polymer chain is confirmed by FT-IR tests. The temperature dependence of FT-IR confirms that the synthesized copolymer has a space length dependent reversible crystallinity. Data from differential scanning calorimetry (DSC) also shows that the hard segment crystallization is strong and reversible in nature. The XRD results show that the polymer is semi-crystalline. The TGA analysis confirmed that the synthesized copolymers are thermally stable up to 290°C. The presence of hydrogen bonds in polymer chains is thermally reversible. The polymer’s solvent resistivity is excellent due to its high crystallinity.

Polydimethylsiloxane‐based Polyurethane/cellulose Nanocrystal Nanocomposites: From Structural Properties Toward Cytotoxicity

Polyurethane/crystalline nanocellulose (PU/CNC) nanocomposites were fabricated through in-situ polymerization by using polydimethylsiloxane (PDMS), methylene diphenyl diisocyanate (MDI), and 1,4-butanediol (BDO) as polyol, aromatic diisocyanate, and chain extender, respectively. CNC content affected the microstructure and thermophysical properties of the nanocomposites. Incorporation of CNCs into PU affected the interactions between soft and hard segments. Although CNCs had slight effect on thermal degradation of nanocomposites, they significantly affected thermophysical properties. For example, CNCs decreased the crystallization of hard segment and glass transition temperatures of hard and soft segments were shifted to higher temperatures. PU showed a relatively high toxicity against human fibroblast cells because of presence of aromatic segments in its structure. Although cytocompatibility was improved by addition of CNCs, PU/CNC nanocomposites showed relatively cytotoxic behavior at higher concentrations.

Biodegradable poly(urethane-urea-amide): Synthesis, characterization and mechanical studies

Biodegradable polyurethane with excellent mechanical property finds a lot of applications in the biomedical field. In this study, multi block copolymer poly(urethane-urea-amide) (PUUA) is prepared using polyethylene oxide, 1,6-hexamethylene diisocyanate and diamide (N1, N4-bis(6-aminohexyl)benzene-1,4-dicarboxamide (6T6) or N1, N6-bis(6-aminohexyl)hexanediamide (6A6)) by a method of solution/melt polymerization techniques. The polymer formation is confirmed by the 1H NMR spectroscopic method. Other techniques such as viscosity measurements, FT-IR, DSC, TGA, and biodegradability are used to characterize the synthesized copolymers. The hard segment crystallinity is an analyzed by FT-IR spectroscopy, it revealed that the 6T6 based polymer showed 70% of crystallinity. The synthesized copolymer shows three transitions in the DSC curve. The melting enthalpy of the hard segment depends on the amide unit employed in the polymerization process. These materials are fast crystallizing due to low under cooling value. This material has a very high thermal stability and stable up to 400°C. Tensile test confirms the strain induced crystallization of hard segment occurred in the PUUA. The biodegradability of the polymer depends on the pH and chain extenders. This type of materials can be a good candidate for tissue engineering application.

Size-dependent phase separation and thermomechanical properties of thermoplastic polyurethanes

Over the years, the manipulation of polymer microphase separation associated with molecular architectures has been holding great promise for rendering structure-property relationship. A series of poly (urea/malonamide) dendrons bearing rigid adamantanes in the periphery were developed to serve as branching hard segments (HS) for thermoplastic polyurethanes (TPUs) either in end-capping or pendant manner. Thermal properties, crystalline structures, and mechanical properties of the TPUs were investigated by DSC, X-ray scattering, and DMA, respectively. The introduction of the branching units with bulky adamantanes would expectedly hinder the crystallization of HS. Intriguingly, the dendrons not only accelerate the crystallization of the soft segment (SS, based on polycaprolactone polyol (PCL)), but promote the phase separation between HS and SS to form distinct microdomains as well. These enhanced effects revealed a size-dependent relationship with growing size (or generation) of the branching HS. With the high SS crystallinity that can freeze the chain mobility and the robust HS domains that work as the physical crosslinking points, the resulting TPUs incorporated with adamantane-bearing units show superior shape memory behaviors. This work demonstrates that the molecular engineering on the architectures can significantly tailor the microstructures and properties of thermal plastic polymers.

Evaluation of the engineered polysaccharide alpha‐1,3 glucan in a thermoplastic polyurethane model system

AbstractEnzymatic polymerization is under development as novel scalable process technology to convert sucrose to engineered polysaccharides. Similar to established monomer‐based polymerization processes, this approach allows for the synthesis of glucose‐based polymers with controlled polymer linkage, structure, and material morphology. Using enzymatic polymerization, alpha‐1,3‐polyglucose (glucan) can now be produced from sugar on scales required for industrial applications. This alpha‐1,3 glucan material, with accessible primary and secondary hydroxyl groups within the overall defined particle morphology, is especially of interest as a partially reactive component in polyurethane chemistry. This study explores the impact of alpha‐1,3‐glucan as additive in a thermoplastic polyurethane model system and the improvement in mechanical properties of these composites. Glucan was effectively first mixed with a polyether polyol diol, forming a stable dispersion with narrow particle size distribution, followed by reaction with diisocyanate and chain extender to form the polyurethane matrix. The analysis of the generated polyurethane matrix indicates that the hydroxyl groups of the dispersed glucan particles directly react with isocyanate. Tetrahydrofuran solubility of the formed polyurethane compound decreased with the addition of glucan, providing evidence of covalent bonding of glucan leading to cross‐linking of the polyurethane matrix. Thermal analysis of this model system suggests that the glucan additive induces hard segment crystallization, resulting in increased hardness and tensile modulus compared with the reference. Based on the observed property enhancements, engineered polysaccharides provide a sustainable performance additive for polyurethane materials.

Integrated Computational and Experimental Design of Ductile, Abrasion-Resistant Thermoplastic Polyurethane/Graphene Oxide Nanocomposites

Polymer nanocomposites are widely studied for improving and developing novel materials. Incorporation of nanofillers in polymer matrices impart strong behavioral changes, with the extent of dispersion of fillers in polymers playing a key role. This not only limits the amount of filler one can incorporate but also often leads to enhancement of some material properties at the expense of others. Herein, for the first time, thermoplastic polyurethane (TPU) graphene oxide (GO) nanocomposites with improved abrasion resistance and ductility are produced by integrating mesoscale modeling and a solvent-free continuous and upscalable extrusion process. The role of GO in hard segment crystallization is established via dissipative particle dynamics simulations, which then informs processing in twin-screw extrusion involving extensional mixing elements to achieve desired deagglomeration and dispersion of GO. This approach allows a tough yet highly ductile composite suitable for high abrasion resistant applications to be produced. In comparison with composites obtained from conventional processing, ductility improved by more than 300%, strength increased by 80%, toughness enhanced by more than 500%, and abrasion resistance improved by 45%. Insights into the gradient of TPU hard block crystallinity, role of deagglomeration, and phase separation are also discussed.

Transparency‐tunable and moderate‐temperature healable thermoplastic polyurethane elastomer based on bisphenol A chain‐extender

AbstractTransparent self‐healing polymers have a promising future, but the research on the relationship among transparency, self‐healing, and structure is still challenging. The thermoplastic polyurethane elastomers containing aryl carbamate were prepared by the reaction of isocyanate‐terminated prepolymer and chain‐extender bisphenol A (BPA). On the one hand, the thermoplastic polyurethane elastomer could heal at a relatively low temperature due to the exchange of aryl carbamate, and exhibits 83% of healing efficiency at 90°C for 2 h. On the other hand, two methyl groups of BPA hinder the crystallization of hard segments in polyurethane, so the domain of hard segments is amorphous, while the microcrystals of soft segments dispersed in the polyurethane are affected by the content of hard segments, therefore, the transparency of polyurethane can be tailored by the content of hard segments. It provides a simple design and a feasible preparation method for transparent self‐healing polymer.

Microcellular foaming behavior of ether- and ester-based TPUs blown with supercritical CO2

Abstract The bead foaming behavior of ether- and an ester-based Tensor Processing Unit (TPU) resins were investigated in a lab-scale reactor using supercritical CO2 as the blowing agent. The samples were saturated at various saturation temperatures and the effects of hard segment crystallization during the saturation on the foaming behavior of the TPU samples were explored. The results revealed that the different HS crystallization tendencies and possible CO2 solubility differences in two TPU grades led to their different foaming behaviors. The ester-based TPU could be foamed within a wider saturation temperature range and revealed an easier cell growth and foam expansion while the ether-based TPU showed a more limited cell growth behavior and hence processing window. The effect of pre-annealing and hence the isothermally induced HS crystallization on the foaming behavior of the ether-based TPU and the influence of depressurization rate on the foaming behavior of ester-based TPU was also explored.

Effect of soft segment molecular weight on the microcellular foaming behavior of TPU using supercritical CO2

This study investigates the foaming behavior of TPU samples with three different soft segment (SS) molecular weights of 1000, 2000, and 3500 g/mol. The variation of SS length did not only enhance the flexibility of the TPU samples but could also ease the hard segment (HS) crystallization with various size/closed packed structure with much wider HS crystal melting peak. Such broadened melting peak expanded the foam processing window in TPUs with higher SS length. Moreover, the results showed that the increase in SS length could not only enhance the foam expansion behavior but could also enhance the heterogeneous cell nucleation and more importantly minimize the shrinkage of the TPU foams as a result of broadened HS crystallization. The saturation temperature, pressure, and time could also differently influence the formation of HS crystals in each TPU with different structure which could eventually differently affect the TPUs’ foaming behavior.

Effect of hard segment content on the microcellular foaming behavior of TPU using supercritical CO2

A lab-scale reactor was used to investigate the foaming behavior of three different thermoplastic polyurethane (TPU) grades with different hard segment (HS) contents of around 39%, 49% and 57 wt%. The effect of HS content on the foaming behavior of TPU was extensively analyzed while varying the foam processing parameters. The saturation temperature, pressure, and time differently influenced the formation of HS crystals in each TPU with different closed-pack structures and sizes. With the increase in HS crystallization during the saturation the heterogeneous cell nucleation was improved and the foam expansion was controlled by the melt strength of the TPU/CO2 mixture, solubility of CO2, and the final shrinkage of the foamed samples. In TPUs with higher HS content, the processing window was broadened and shifted to higher temperatures while the expansion ratio was limited and shrinkage was minimized.

Crystallization of hard segments in MDI/BD-based polyurethanes deformed at elevated temperature and their dependence on the MDI/BD content

Morphological variation of thermoplastic polyurethanes (TPUs) with different hard segment contents (HSCs) stretched at 100 °C higher than the Tg of hard domain was monitored by in-situ wide-angle X-ray scattering (WAXS) and small-angle X-ray scattering (SAXS) techniques. WAXS patterns reveal that the crystalline modification of TPUs with high HSCs transforms from paracrystal (termed as form I) to form III of 4,4′-methylene diphenyl diisocyanate/1,4-butanediol (MDI/BD) segment, and TPUs with low HSCs show the direct transition from amorphous MDI/BD packing to form III packing. The form III crystals are fibrillar hard domains. Chord distribution functions (CDFs) computed from the SAXS patterns were employed to illustrate the domain topology. The streak-like CDF peaks off the meridian indicate the existence of cylindrical microfibrils which are composed of very low inhomogeneities between the destructured hard domains and stretched soft domains along the straining direction. These microfibrils of TPUs with low HSCs present a periodically lateral correlation. Interface distribution function (IDF) and chord length distribution (CLD) were computed from Bonart's longitudinal and transverse projection to quantify the domain stacking and microfibril arrangement respectively. The average size of hard domains increases up to a constant size with the increasing macroscopic strain which may be related to the maximal sequence length of hard segment. The average size of soft domain also reaches up to a constant size at large strain that is smaller than the fully-extended length of soft segment. The variation of domain size is different from deformation at room temperature. The HSC affects the behaviors of morphological evolution significantly.

Characterization of hard-segment crystalline phase of poly(ether-block-amide) (PEBAX®) thermoplastic elastomers in the presence of supercritical CO2 and its impact on foams

The effects of the saturation temperature and high-pressure dissolved CO2 on the crystallization behavior of polyether-block-amide (Pebax®) containing various amounts of polyamide-12 hard segment were studied. Non-isothermal and isothermal analyses were conducted using differential scanning calorimetry (DSC) to evaluate the crystallization kinetics of Pebax® samples. Wide angle X-ray diffraction (WAXD) was used to characterize the crystalline morphology of Pebax® under various conditions. A higher cell density was observed from the foams of the broader distribution of the hard segment length and the hard segment crystalline phase obtained by twin-screw compounding. It appears that the hard-segment crystals are the heterogeneous nucleating sites that govern the nucleation phenomena of Pebax®. Therefore, the broader distribution of crystalline phase offers better foaming characteristics because of the larger number of nano-/micro-scale heterogeneous nucleation sites.

Fumed silica/polyurethane nanocomposites: effect of silica concentration and its surface modification on rheology and mechanical properties

The effects of nanosilica type and its content on microstructure, mechanical properties, and rheology of thermoplastic polyurethane (TPU) nanocomposites were investigated. Three different types of silica which included: unmodified (Si-Un) and commercially modified with octylsilane (Si-OS) and polydimethylsiloxane (Si-PDMS) with 5, 10, and 15 wt% of all fillers, were prepared by solution casting method. Scanning electron microscopy (SEM) showed that surface treatment of nanosilica with OS and PDMS reduced the aggregation of particles and improved their dispersion at microlevel. The effect of adding nanoparticles on microdomain morphology of TPU was studied by transmission electron microscopy (TEM), infrared spectroscopy (FTIR), and differential scanning calorimetry (DSC). The results demonstrated a relatively good interaction between the hard and soft segments in the presence of treated nanosilica that hindered the crystallization of hard segments in TPU. Thermogravimetric analysis (TGA) and tensile test showed that nanocomposites with treated nanosilica have better thermal stability and mechanical properties. The dynamic rheological studies indicated that nanocomposites containing Si-OS and Si-PDMS (with better dispersion and higher interface between the soft and hard domains in TPU) have improved viscoelastic properties in comparison with nanocomposites with untreated silica. In this study, dynamic frequency sweep data were correlated by a generalized Maxwell model and found that elastic constants of TPU chains were improved in the presence of modified silica nanoparticles.

Primary Chemical Sequence Ultimately Determines Crystal Thickness in Segmented All-Aliphatic Copolymers

Thermoplastic elastomers (TPEs) are known to exhibit a phase-separated morphology which depends on their chemical structure and processing. The design of novel TPEs with predefined properties which are also independent of the material thermal history has so far remained a challenge. The focus of this work is on the semicrystalline morphology of all-aliphatic thermoplastic elastomers consisting of alternating polytetrahydrofuran (PTHF) segments and uniform glycine or β-alanine bisoxalamide units. The thickness of the hard segment crystals was found to be highly monodisperse and independent of the sample thermal history. Using Nanocalorimetry, we observed that at cooling rates as high as 12 000 °C s–1 the bisoxalamide segments can still crystallize although the crystallization temperature decreases by ca. 26 °C. The surface free energy of the hard block crystals is found to be extremely low (∼18 mJ·m−2), which is likely due to the entropic contribution of soft segments forming tie chains bridging the neighboring crystals. To investigate the combined effect of crystal orientation and phase transitions, simultaneous time-resolved X-ray scattering and mechanical tensile tests were performed. Upon stretching, elastomeric PTHF segments with lengths above 1000 g mol–1 crystallize at ambient temperatures. Under these conditions two main morphologies were observed: at low strains the long axes of the fibril-like crystals were oriented parallel to the flow direction, whereas higher strains caused bisoxalamide crystal fragmentation and changed their preferential direction to the one perpendicular to the drawing direction. The chain tilts in the bisoxalamide crystals were calculated from the characteristic four-spot SAXS patterns and were ∼5°–16° in the case of glycine end-groups and 24° for alanine and propyl terminal groups. To our knowledge, this is the first attempt to determine the chain tilt for the nonlamellar crystals in block copolymers.

Crystallization of hard segment domains with the presence of butane for microcellular thermoplastic polyurethane foams

This paper presents the effects of dissolved butane on the crystallization of hard segment (HS) and its effects on enhancement of cell nucleation to manufacture microcellular thermoplastic polyurethane foams (TPU). The TPU samples were first reprocessed by melt compounding through a twin-screw extruder. A broad distribution of HS domains was generated after this processing, including some perfected HS nano-crystals with a higher melting temperature. In addition to the processing effect, the saturation temperature and the plasticizing butane significantly induced a larger number of HS crystalline domains. Consequently, cell nucleation was significantly promoted in the vicinity of the largely distributed HS crystalline domains over a wider saturation temperature range of 150 °C–170 °C at a saturation pressure of 55 bar.

Influence of the content of hard segments on the properties of novel urethane-siloxane copolymers based on a poly(ε-caprolactone)-b-poly(dimethylsiloxane)-b-poly(ε- caprolactone) triblock copolymer

A series of novel thermoplastic urethane-siloxane copolymers (TPUSs) based on a ?,?-dihydroxy-[poly(?-caprolactone)-bpoly( dimethylsiloxane)-b-poly(?-caprolactone)] (?,?-dihydroxy-PCLPDMS- PCL) triblock copolymer, 4,4?-methylenediphenyl diisocyanate (MDI) and 1,4-butanediol (BD) was synthesized. The effects of the content (9-63 wt. %) of hard urethane segments and their degree of polymerization on the properties of the segmented TPUSs were investigated. The structure, composition and hard segment degree of polymerization of the hard segments were examined using 1H- and quantitative 13C-NMR spectroscopy. The degree of crystallinity of the synthesized copolymers was determined using wide-angle X-ray scattering (WAXS). The surface properties were evaluated by measuring the water contact angle and water absorption. In the series of the TPUSs, the average degree of polymerization of the hard segments was varied from 1.2 to 14.4 MDI-BD units. It was found that average values from 3.8 to 14.4 MDI-BD units were effective segment lengths for crystallization of hard segments, which resulted in an increase in the degree of microphase separation of the copolymers. Spherulite-like superstructures were observed in copolymer films by scanning electron microscopy (SEM), which are believed to arise from the crystallization of the hard segments and/or PCL segments, depending on the content of the hard segments. The surface of the copolymers became more hydrophobic with increasing weight fraction of PDMS. The synthesized copolymers based on a PCL-PDMS-PCL segment showed good thermal stability, which increased with increasing content of soft PDMS segments, as was confirmed by the value of the starting temperature of thermal degradation.