Hard Segment Structure Research Articles

Role of the Soft and Hard Segments Structure in Modifying the Performance of Acrylic Adhesives Modified by Polyurethane Macromonomers.

Acrylic pressure sensitive adhesives, modified by polyurethane (PU), achieve selective optimization through the designability of polyurethanes. In this paper, PU macromonomers were prepared by a two-step synthesis method, using polypropylene glycol or polyethylene glycol with different molecular weights as soft segments and different types of diisocyanates: isophorone diisocyanate, hexamethylene diisocyanate, dicyclohexylmethylmethane diisocyanate (HMDI), 2,4-toluene diisocyanate (TDI), and 4,4'-diphenylmethane diisocyanate (MDI) and chain extenders as hard segments. After being terminated by capping agents, a series of PU macromonomers of different molecular weights and structures were obtained and used to modify the acrylic base adhesives. Compared to unmodified adhesives, acrylic adhesives modified by PU macromonomers have improved adhesion performances and heat resistance and show an increasing trend with the increase of molecular weight of diols. Diols with a molecular weight of 600 have the best effect. Diisocyanates containing benzene rings can better improve the thermal performance of adhesives, where P MDI containing a biphenyl ring is the best, while aliphatic isocyanate groups have a greater improvement in adhesion performance, and the adhesion performance of P HDI with a long carbon chain is the best.

Effect of hard segment chemistry and structure on the self‐healing properties of UV‐curable coatings based on the urethane acrylates with built‐in Diels–Alder adduct

AbstractThe development of new photoreactive resins to obtain smart coatings and enhance the self‐healing (SH) properties of photoreactive resins has attracted great interest in the past few years, and the investigation of the relationship between phase structure and smart properties is necessary. Herein, four types of urethane acrylate resins with built‐in DA structures with different hard segment structures, and a slight change in the length of the polyol soft segment were prepared. The hard segments were made of hexamethylene diisocyanate (HDI) with a linear structure and isophorone diisocyanate (IPDI) with an alicyclic structure, and the soft segments consisted of polyethylene glycols (PEG 400 and PEG 1000, respectively). The obtained resins were used to prepare polymer films that were cured using UV radiation. The coatings obtained in this way were tested for the influence of the polymer chain architecture on the kinetics of photopolymerization, the properties of the cured coatings, as well as the thermal characteristics and thermoreversibility performance of the cured coatings and their SH ability.

Synthesis and properties of water‐dispersible polyurethanes based on various diisocyanates and PEG as the hard segment

AbstractWater‐dispersible polyurethanes are a greener alternative to polyurethane solutions in organic solvents. Various polyurethane materials were prepared by a two‐step polymerization procedure of polypropylene glycol with Mn of 2000, polyethylene glycol with Mn of 1500, and diisocyanates with different chemical structures, aliphatic, cycloaliphatic and aromatic. It was investigated how the diisocyanate structure affected the molecular morphology, contact angle measurements, and physical characteristics of water‐dispersible polyurethanes. FTIR and NMR measurements showed a close relationship between the intermolecular connections and the hard segment structure, mainly of the diisocyanate used. Also, the polarized light method showed that the presence of the PEG in the hard domain improved aggregation through physical interactions due to its structural flexibility and mobility. These water‐dispersible polyurethanes can assure the use of these polyurethane materials in a broad range of applications with increased environmental protection.

Preparation and relationship between structure and properties of PTMEG based polyurethane acrylate UV cured films

A series of polyurethane acrylate UV curing films (FPUA) were designed and prepared from three different hard segment structure, hard segment content and NCO/OH molar ratio via prepolymer process from polytetramethylene ether glycol(PTMEG), diisocyanate and polycaprolactone modified acrylate (FM1D). And the influence of structural parameters on the cured films’ mechanical properties was studied; Besides, its thermal properties was characterized by Dynamic Thermomechanical Analysis (DMA) and Thermo Gravimetric Analyzer (TGA). Among these three hard segment structures, hexamethylene diisocyanate (HMDI) -based cured films have the best mechanical properties. The modulus and tensile strength of the cured films increases along with the increase of hard segment content, whereas the elongation at break of the cured films decreases since the rising of the NCO/OH molar ratio. Indeed, with hard segment content at 48.6% and NCO/OH molar ratio at 1.06, the UV cured films (exhibits the best performance)with tensile strength up to 50.5 MPa, and break elongation upto238%. DMA and TGA data showed that the low-temperature glass transition temperature of UV-cured films is around −50 °C, and the fastest thermal degradation temperature is 355 °C, showing good resistance to high and low temperatures.

One-Shot Synthesis of Thermoplastic Polyurethane Based on Bio-Polyol (Polytrimethylene Ether Glycol) and Characterization of Micro-Phase Separation.

In this study, a series of bio-based thermoplastic polyurethane (TPU) was synthesized via the solvent-free one-shot method using 100% bio-based polyether polyol, prepared from fermented corn, and 1,4-butanediol (BDO) as a chain extender. The average molecular weight, degree of phase separation, thermal and mechanical properties of the TPU-based aromatic (4,4-methylene diphenyl diisocyanate: MDI), and aliphatic (bis(4-isocyanatocyclohexyl) methane: H12MDI) isocyanates were investigated by gel permeation chromatography, Fourier transform infrared spectroscopy, atomic force microscopy, X-ray Diffraction, differential scanning calorimetry, dynamic mechanical thermal analysis, and thermogravimetric analysis. Four types of micro-phase separation forms of a hard segment (HS) and soft segment (SS) were suggested according to the [NCO]/[OH] molar ratio and isocyanate type. The results showed (a) phase-mixed disassociated structure between HS and SS, (b) hydrogen-bonded structure of phase-separated between HS and SS forming one-sided hard domains, (c) hydrogen-bonded structure of phase-mixed between HS, and SS and (d) hydrogen-bonded structure of phase-separated between HS and SS forming dispersed hard domains. These phase micro-structure models could be matched with each bio-based TPU sample. Accordingly, H-BDO-2.0, M-BDO-2.0, H-BDO-2.5, and M-BDO-3.0 could be related to the (a)—form, (b)—form, (c)—form, and (d)—form, respectively.

Room-temperature self-healing polysiloxane elastomer with reversible cross-linked network

The linear self-healing matrix had certain defects in the aspects of physical and mechanical properties, heat resistance and solvent resistance, which limited its application in engineering. However, if the conventional cross-linked structure was integrated into material matrix, the self-healing properties of the material would be sacrificed seriously. To address the drawbacks of linear and cross-linked self-healing polymers, reversible cross-linked structural units were designed to balance the two aspects. The silicone elastomers were endowed with outstanding mechanical properties and self-healing ability at room temperature by introducing a tri-functional cross-linking agent “Trimer of hexaethylene-diisocyanate (Tri-HDI)” to PDMS linear structure, and distributing dynamic disulfide bonds around of the cross-linking points to build the reversible cross-linking structural units. The influence of cross-linker content on the mechanical properties, self-healing properties and heat resistance of the elastomer was systematically investigated in this paper. Then, the soft and hard segment structures at different temperatures of the self-healing materials was characterized by low-field NMR as an innovative structural characterization. This room temperature self-healing elastomer with reversible cross-linked structure had great potential for industrial applications.

Ultra-high-strength self-healing supramolecular polyurethane based on successive loose hydrogen-bonded hard segment structures

Solving the weak toughness and strength of self-healing materials seems to be a mountain that is difficult to get over. Although some studies have reported high-strength self-healing materials, human intervention and long heal time are needed as compensation to self-heal. The balance between mechanical properties and self-healing efficiency is still challenging. Here, a high-strength self-healing supramolecular polyurethane is constructed based on a continuous loose hard segment structure. The polyurethane exhibits a maximum tensile strength greater than 45 MPa, maximum elongation at break of 850 %, and a tear strength of up to 118.776 KJ/m3, a self-healing efficiency of 95 %, which can effectively prolong the service life of the material. Strong soft segments and high-density loose hydrogen bond arrays are the source of strong mechanical strength. The asymmetric alicyclic loose hard segment structure ensures sufficient mobility of the molecular segments to complete hydrogen bond recombination in a short time. At the same time, its research as a conductive polymer matrix has been verified to restore 90 % of the conductivity, showing great application potential in the field of self-healing conductive composite material.

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.

Study on properties of polyurethane elastomers prepared with different hard segment structure

AbstractFour different systems of cast polyurethane elastomers were prepared through two‐step method by adjusting different isocyanates and amine chain extenders. The blocked polyurethane prepolymer synthesized by the reaction of methyl ethyl ketoxime (MEKO) and the prepolymer weakens the reactivity with the amine chain extender. The mechanical and thermal properties of cast polyurethane prepared by different combinations were analyzed through comparison. The microphase separation of hard and soft segments structure in different systems were observed by using atomic force microscope (AFM). Through dynamic mechanical analysis (DMA), the influence of various structures on the loss factor is discussed. Studying the dynamic endogenous heat of cast polyurethane (CPU) by changing the hard segment structure provides a novel idea for the construction of polyurethane with low‐damping structure.

Evolution of ordered structure of TPU in high-elastic state and their influences on the autoclave foaming of TPU and inter-bead bonding of expanded TPU beads

Different from the melt foaming of extrusion and injection molding, polymer is in high-elastic state during autoclave foaming. In this study, the critical parameters used to describe the high-elastic state of polymer, i.e., softening temperature (Ts) and viscous flow temperature (Tv), were introduced to explain the autoclave foaming of thermoplastic polyurethane (TPU) and the inter-bead bonding mechanism of expanded TPU (ETPU) beads. Evolution of ordered structure of TPU in high-elastic state was characterized by different methods. The steam-chest molding was used to manufacture the ETPU parts, and the molding temperature window was compared with the Ts and Tv of TPU resin, and the endothermic peaks of ETPU beads. The melting of ordered hard segments structure, interface-diffusion of polymer chains, cooling-induced formation of new ordered structure among the diffused polymer chains were the possible mechanism of strong inter-bead bonding in the molded ETPU parts.

Polyimide-Based Membrane Materials for CO2 Separation: A Comparison of Segmented and Aromatic (Co)polyimides.

A series of new poly(ethylene oxide) (PEO)-based copolyimides varying in hard segment structure are reported in this work as CO2 selective separation membranes. Their structural diversity was achieved by using different aromatic dianhydrides (4,4′-oxydiphthalic anhydride (ODPA), 4,4’-(hexafluoroisopropylidene)diphthalic anhydride (6FDA)) and diamines (4,4′-oxydianiline (ODA), 4,4′-(4,4′-isopropylidene-diphenyl-1,1′- diyldioxy)dianiline (IPrDA), 2,3,5,6-tetramethyl-1,4-phenylenediamine (4MPD)), while keeping the content of PEO (2000 g/mol) constant (around 50%). To get a better insight into the effects of hard segment structure on gas transport properties, a series of aromatic polyimides with the same chemistry was also studied. Both series of polymers were characterized by 1HNMR, FTIR, WAXD, DSC, TGA, and AFM. Permeabilities for pure He, O2, N2, and CO2 were determined at 6 bar and at 30 °C, and for CO2 for pressures ranging from 1 to 10 bar. The results show that OPDA-ODA-PEO is the most permeable copolyimide, with CO2 permeability of 52 Barrer and CO2/N2 selectivity of 63, in contrast to its fully aromatic analogue, which was the least permeable among polyimides. 6FDA-4MPD-PEO ranks second, with a two times lower CO2 permeability and slightly lower selectivity, although 6FDA-4MPD was over 900 times more permeable than OPDA-ODA. As an explanation, partial filling of hard domain free voids by PEO segments and imperfect phase separation were proposed.

Computational development and validation of a representative MDI-BDO-based polyurethane hard segment model.

Segmented polyurethanes show extraordinary physicochemical properties, mainly owing to the nature and the chemistry of the hard segment domains. There are yet many inexplicable physiochemical properties of MDI-BDO-based hard polyurethane segments such as the geometry, cis-trans isomerism, electronic structure, chemical reactivity, the inter-hard-segment interactions, and the photo-response. In the present study, it was attempted to develop and validate a model system that would facilitate further research on the structural and chemical properties of the MDI-BDO hard segments. It was found that the trans isomer of urethane bond is more stable than the cis isomer, and it is argued here that thermal transformation from trans to cis not possible due to the high rotational energy barrier. The differences between the calculated IR spectra of the cis and trans isomers are proposed as a powerful differentiation tool. The calculated Fukui indices show that cis and trans isomers are different in their chemical reactivity. The findings of the present study suggest intermolecular and intramolecular pi-stacking and highly plausible two significant types of hydrogen bond types between hard segments. In the present study, a model system for MDI-BDO hard segment was developed and successfully validated via computational experiments. Further calculations done with the new model provided an indispensable understanding of the structure, cis-trans isomerism, reactivity, and intermolecular interactions of the MDI-BDO hard segments. The proposed model can be further improved in the future by incorporating suitable soft segments. In summary, the model system developed and validated in the present study has provided new opportunities to understand and further study the structural and chemical features of the hard segments of the MDI-BDO-based polyurethane.

Polyurethane as smart biocoatings: Effects of hard segments on phase structures and properties

Development of smart polyurethane (PU) coatings for enhancing the biocompatibility of the metal scaffold has attracted great interest in the past few years, and the investigation of the relationship between phase structure and smart properties is necessary. Herein, two series of PU biocoatings with different hard segment structures and soft segment length were prepared, and the soft segments consist of the polycaprolactone diols (PCL) with different molecular weights, and the hard segments were made of 1, 4-butanediol (BDO) and hexamethylene diisocyanate (HDI) with linear structure, or BDO and isophorone diisocyanate (IPDI) with alicyclic structure, respectively. To understand the phase structures, the chemical structure, surface micromorphology, and segmental incompatibility of the PU films were characterized, which reveals that IPDI-based PU possesses a phase compatible structure with a single-phase transition above 0 °C. Besides, the IPDI-based PU shows the thermal-responsive deformation at the temperature of 45 °C, slightly higher than the human body temperature. In contrast, the HDI-based PU possesses the excellent shape memory property caused by the microphase separation structure, because of the crystallization of soft and hard segments from the melt. Further, based on the characteristics of phase structure, the potential application of the IPDI-based PU and HDI-based PU is explored, offering a useful approach to effectively design the structures and properties of PU coatings.

Influence of chemical structure of hard segments on physical properties of polyurethane elastomers: a review

Hard segments of polyurethanes (PUs) are generally formed from diisocyanate and diol. Diol can be replaced with triol and thiol. These chemical structures of hard segments strongly affect not only a microphase separated structure but mechanical properties of resultant PUs. In this review, we focus on the relationship between chemical structure like symmetry and bulkiness of diisocyanate and mechanical properties of PU. Then, influence of hard segment content, incorporation of 1,1,1-trimethylol propane with trifunctional groups, and alkyldithiol was reviewed mainly on trans-1,4-bis(isocyanatomethyl) cyclohexane-poly(oxytetramethylene) glycol-based PU.

Influence of Polyol/Crosslinker Blend Composition on Phase Separation and Thermo-Mechanical Properties of Polyurethane Thin Films.

Polyurethanes (PUs) from Polyethylene glycol (PEG) and polycaprolactone diol (PCL) and a crosslinker, Pentaerythritol (PE), were synthetized with isophorone diisocyanate (IPDI). In this study, we investigated the effect of polyol and crosslinker composition on phase separation and thermo-mechanical properties. The properties were studied through dynamic mechanical analysis, X-ray scattering, atomic force microscopy (AFM), and thermogravimetric analysis (TGA). The results showed changes in PUs properties, microphase structure, and separation due to the composition of polyol/crosslinker blend. So, the largest concentration of PE produced multimodal loss factor patterns, indicating segment segregation while PUs with a PEG/PCL = 1 displayed a monomodal loss factor pattern, indicating a homogeneously distributed microphase separation. Additionally, the increase of the PEG concentration enhanced the damping capacity. On the other hand, agglomeration and thread-like structures of hard segments (HS) were observed through AFM. Finally, the thermal behavior of PUs was affected by chemical composition. Lower concentration of PE reduced the crosslinking; hence, the temperature with the maximum degradation rate.

Robust Stretchable Thermoplastic Polyurethanes with Long Soft Segments and Steric Semisymmetric Hard Segments

Thermoplastic polyurethane (TPU) elastomers based on polyoxytetramethylene glycol (PTMG) with 2000 g·mol–1 as soft segments (SSs) and isocyanates as well as their reaction with butanediol as hard segments (HSs) were synthesized. The three-dimensional supramolecular networks were constructed in TPU by the formation of physical HSs-rich domains via microphase separation, hydrogen bonding, and microcrystallization, which are dominated by the chemical structure of HSs. The robust stretchable TPU with an excellent tensile strength of 32 MPa and elongation of 1378% at break could be successfully prepared, and no crystallization happened to keep the elasticity even at a low temperature (−90 °C) when the steric semisymmetric bis(4-isocyanatocyclohexyl)methane (HMDI) was introduced into HSs-rich domains. The crystallinity and ordered hydrogen bonding content decreased in HSs-rich domains with increases in steric hindrance and asymmetry of diisocyanates. A new approach was developed to construct the high-performanc...

Thermal Resistance Properties of Polyurethanes and its Composites: A Short Review

The nature of starting materials and the conditions of polyurethane (PU) preparation are regarded as the main general parameters that determine PU thermal resistance. The effect of structure and presence of additives were identified as the major general factors on this regard. Structural factors include phase microstructure, i.e. chemical structure, proportion and segregation of soft and hard segments); polyol type (petrochemical or natural oil-based); isocyanate and chain extender type and thermoplasticity of PU. Respect to the effect of additives, the incorporation of fillers is the most direct strategy to increase PU heat resistance. With respect to fiber additives, in general a positive effect is found on improving thermal resistance, although this generalization could not apply, considering the large number of different PU and environmental conditions of usage.

Linear and nonlinear viscoelastic properties of segmented silicone-urea copolymers: Influence of the hard segment structure

The linear and non-linear viscoelastic behaviors of 5 segmented silicone copolymers - with low fraction of 1,6-hexamethylene diisocyanate (HDI), 4,4′-methylenebis (cyclohexyl isocyanate) (HMDI), 1,3-bis(1-isocyanato-1-methylethyl)benzene (TMXDI), 2,4-tolylene diisocyanate (TDI) or isophorone diisocyanate (IPDI) as hard segments (HS) - are compared, in relation with their microstructure. When the HS are non-symmetrical, the materials nanostructuration is weak and has a limited impact on the mechanical response: two mechanical relaxations occur after the α relaxation of the polydimethylsiloxane (PDMS), the first related to the “unfreezing” of the HS-HS bonds, and the second one to the sticky reptation of the polymer chains. Conversely, when the HS are symmetrical, their self-organization is favored and the long range ordering of the HS is possible. The materials are then semi-crystalline like, with a low crystallinity (due to the low weight percentage of HS) and their flow, possible above their melting temperature (Tm), depends also on the HS dynamics, which is the most rapid for HDI.

The effect of hard segments structure on the functional properties of polyurethane elastomers based on oligoesterurethane epoxides

A series of polyurethane elastomers based on polyester urethane epoxy oligomers and cycloaliphatic diamines was synthesized. The relationship of the physico-mechanical properties of these elastomers with the structure of their polymer chain was studied. It was established that various options for increasing the strength of the final elastomers can be implemented, depending on the type of initial oligoesters and diamines. They may be associated with an increase in the degree of microphase separation between hard and soft polymer segments, and with partial crystallization of soft segments as well.

Design-properties relationships of polyurethanes elastomers depending on different chain extenders structures

In this study, the effect of the structure and amount of the chain extenders on the morphological image and physico-chemical properties of some new polyurethane elastomers has been investigated. To achieve this, three series of polyurethane elastomers based on poly(tetramethylene ether) glycol, hexamethylene diisocyanate and chain extenders with different structures (triethylene glycol, 3,6-dithia-1,8-octanediol, 1,6-hexanediol) were synthesized. The chain-extenders which introduce oxygen or sulfur atoms into the polyurethane backbone chains (hard domains) change the behavior of the properties compared to the corresponding polyurethanes chain-extended with aliphatic diols. The structures of the new polyurethane elastomers were examined by FTIR, X-ray diffraction analysis and by atomic force microscopy (AFM). They were also characterized for thermal and tensile properties. The polyurethanes with sulfur into their hard segment structure were found to exhibit improved thermal stability properties and equivalent mechanical properties with polyurethanes obtained with aliphatic diols. This is due to the extensive and many fold hydrogen bond network that characterizes polyurethanes with sulfur in their hard segment structure.