Patent Application
20160102168
This application relates to semi-crystalline thermoplastic polyester urethanes with controlled concentration, distribution, and types of crystalline hard segment blocks to correlate the effect of hard segment crystallinity to that of the soft segment blocks.
Growing concerns over the environmental impacts of non-biodegradable plastic waste and the need for sustainability have stimulated research efforts on biodegradable polymers from renewable resources. Rising costs and dwindling petrochemical feedstocks also make renewable resource-based materials attractive alternatives to their petroleum-based counterparts.
Segmented thermoplastic polyester urethane (TPEU) elastomers have attracted significant interest because they generate a wide variety of industrial applications ranging from foams and coatings to medical devices, where the hydrolytically labile polyester functions provide controlled degradation. TPEUs may possess the structure (—X—Y—)n, composed of a polyester macro diol, soft segment (SS) block, and urethane rich, hard segment (HS) block. Their versatility stems from the chemical compositions of X and Y units. In conventional TPEU elastomers, the incompatible X and Y units phase separate into nano scale domains of amorphous HS that serve as the load bearing phase in the rubbery soft polyester phase which imparts extensibility.
Research interest on crystalline SS and HS block TPEUs has seen a surge recently, especially due to their potential shape memory properties. Crystallinity of SS-block is observed for sufficiently long macro diols. A moderate soft segment crystallinity in TPEUs leads to increased incompatibility between the hard and soft domains, and enhanced the mechanical performance. Accordingly, numerous studies to tune the ordering of soft segment blocks have been undertaken. This includes varying the type of soft segment, their size, content, introducing side chain liquid crystal soft segments, etc. A systematic conceptual understanding of the role of crystalline HS-blocks in controlling the SS-block crystallinity, however, is limited since the majority of commercial TPEUs do not exhibit hard segment crystallinity. The lack of molecular symmetry for the industrially available diisocyanate molecules and the low molecular weight of chain extenders limited the crystallization of hard segments in commercial TPEUs. However, aliphatic hexamethylene diisocyanate (HMDI) have been shown to offer enhanced ordering of the hard segment and to prevent the hydrolytic degradation of ester groups in poly(ester urethane) elastomers.
TPEUs synthesized from renewable resources have been receiving increased attention due to a perceived need to reduce petroleum dependence and address negative impacts on the environment. A significant amount of that attention has focused on the use of vegetable oil derived feedstock, due to their relative availability, flexibility with regards to chemical modification, low toxicity and inherent biodegradability. Numerous studies have been carried out to develop diols or polyols suitable for polyurethane production from vegetable oils, to entirely or partially replace conventional petroleum-based materials, with a certain degree of success realized. Efforts to synthesize di-isocyanates from vegetable oils have been limited compared to those focused on polyols, but some progress has been made. These have included: (i) synthesis of fatty acid based di-isocyanates; (ii) C36 fatty acid based diisocyanates; and (iii) soybean oil based polyisocyanate prepared via a vinyl bromination of triglycerides followed by substitution with AgNCO. More recently, di-isocyanates were prepared at the lab scale from fatty acid derived diamines using a phosgene method, or directly from fatty acids using Curtius rearrangement. Thermoplastic polyurethanes have been prepared from these fatty acid derived di-isocyanates by combination with either petroleum-based or bio-based diols. However, the resulting materials displayed low molecular weights due to the low chemical reactivity of fatty acid based diisocyanates, particularly 1,7-heptamethylene diisocyanate (HPMDI), produced from Curtius rearrangement of fatty diacids.
The poor performance of HPMDI based thermoplastics have motivated the current effort, which focuses on the optimization of the polymerization reaction conditions, and selection of suitable polyester macro diol and chain extenders in order to develop high molecular weight semi-crystalline TPEU elastomers with varying chemical compositions of the HS and SS-blocks. A series of TPEUs were prepared from a vegetable-oil based di-isocyanate, chain extenders and a petroleum-based polyester macro diol, using varying polymerization protocols. The TPEUs were chemically and physically characterized. The effects of HS-block content, distribution and type on thermal stability, melting and crystallization behavior and mechanical properties were investigated.
versus
for S1 series TPEUs having HS-blocks of different lengths (x=1-5). The line is a linear fit (R2>0.9801).
The synthesis of certain thermoplastic polyester urethanes having crystallizable hard segments and soft segments were prepared from the following materials: (i) a natural oil based organic isocyanate, (ii) a diol component, and (iii) and a chain extender.
As used herein, the term “natural oil” may refer to oil derived from plants or animal sources. The term “natural oil” includes natural oil derivatives, unless otherwise indicated. Examples of natural oils include, but are not limited to, vegetable oils, algae oils, animal fats, tall oils, derivatives of these oils, combinations of any of these oils, and the like. Representative non-limiting examples of vegetable oils include canola oil, rapeseed oil, coconut oil, corn oil, cottonseed oil, jojoba oil, olive oil, palm oil, peanut oil, safflower oil, sesame oil, soybean oil, sunflower oil, linseed oil, palm kernel oil, tung oil, jatropha oil, mustard oil, camelina oil, pennycress oil, hemp oil, algal oil, and castor oil. Representative non-limiting examples of animal fats include lard, tallow, poultry fat, yellow grease, and fish oil. Tall oils are by-products of wood pulp manufacture. In certain embodiments, the natural oil may be refined, bleached, and/or deodorized. In some embodiments, the natural oil may be partially or fully hydrogenated. In some embodiments, the natural oil is present individually or as mixtures thereof.
Natural oils may include triglycerides of saturated and unsaturated fatty acids. Suitable fatty acids may be saturated or unsaturated (monounsaturated or polyunsaturated) fatty acids, and may have carbon chain lengths of 3 to 36 carbon atoms. Such saturated or unsaturated fatty acids may be aliphatic, aromatic, saturated, unsaturated, straight chain or branched, substituted or unsubstituted, fatty acids, and mono-, di-, tri-, and/or poly-acid variants, hydroxy-substituted variants, aliphatic, cyclic, alicyclic, aromatic, branched, aliphatic- and alicyclic-substituted aromatic, aromatic-substituted aliphatic and alicyclic groups, and heteroatom substituted variants thereof. Any unsaturation may be present at any suitable isomer position along the carbon chain to a person skilled in the art.
The natural oil based organic isocyanate compounds for TPEUs are di-functional isocyanates. The natural oil based organic isocyanates of the described herein have a formula R(NCO)n, where n is 1 to 10, and at times equal to 2, and wherein R includes 2 and 40 carbon atoms, and wherein R contains at least one aliphatic, cyclic, alicyclic, aromatic, branched, aliphatic- and alicyclic-substituted aromatic, aromatic-substituted aliphatic and alicyclic group. Examples of such isocyanates include, but are not limited to, diphenylmethane-4,4′-diisocyanate (MDI), which may either be crude or distilled; toluene-2,4-diisocyanate (TDI); toluene-2,6-diisocyanate (TDI); methylene bis (4-cyclohexylisocyanate (H12MDI); 3-isocyanatomethyl-3,5,5-trimethyl-cyclohexylisocyanate (IPDI); 1,6-hexane diisocyanate (HDI); naphthalene-1,5-diisocyanate (NDI); 1,3- and 1,4-phenylenediisocyanate; polyphenylpolymethylenepolyisocyanate (PMDI); m-xylene diisocyanate (XDI); 1,4-cyclohexyl diisocyanate (CHDI); isophorone diisocyanate; 1,7-heptamethylene diisocyanate (HPMDI); isomers and mixtures or combinations thereof. At times, the natural oil based isocyanate is 1,7-heptamethylene diisocyanate (HPMDI).
The diol component used in the TPEUs are polyester diols. The diols may include hydroxyl-terminated reaction products of dihydric alcohols such as ethylene glycol, propylene glycol, diethylene glycol, neopentyl glycol, 1,4-butanediol, furan dimethanol, cyclohexane dimethanol or polyether diols, or mixtures thereof, with aliphatic dicarboxylic acids (e.g., having 4 to 16 carbon atoms) or their ester-forming derivatives, for example succinic, glutaric and adipic acids or their methyl esters, phthalic anhydride or dimethyl terephthalate. At times, the polyester diol is poly(ethylene adipate) diol, poly(ethylene succinate) diol, poly(ethylene sebacate) diol, poly(butylene adipate) diol, and also at times, poly(ethylene adipate) diol (PEAD).
The chain extenders used in the TPEUs are low-molecular weight compounds containing at least two moieties selected from hydroxyl groups, primary amino groups, secondary amino groups, and other active hydrogen-containing groups reactive with an isocyanate group. Chain extenders include, for example, polyhydric alcohols (especially trihydric alcohols, such as glycerol and trimethylolpropane), polyamines, and combinations thereof. Non-limiting examples of polyamine chain extenders include diethyltoluenediamine, chlorodiam inobenzene, diethanolamine, diisopropanolamine, triethanolamine, tripropanolamine, 1,6-hexanediamine, and combinations thereof. The diamine crosslinking agents include twelve carbon atoms or fewer, more commonly seven or fewer. Other cross-linking agents include various tetrols, such as erythritol and pentaerythritol, pentols, hexols, such as dipentaerythritol and sorbitol, as well as alkyl glucosides, carbohydrates, polyhydroxy fatty acid esters such as castor oil and polyoxy alkylated derivatives of poly-functional compounds having three or more reactive hydrogen atoms, such as, for example, the reaction product of trimethylolpropane, glycerol, 1,2,6-hexanetriol, sorbitol and other polyols with ethylene oxide, propylene oxide, or other alkylene epoxides or mixtures thereof, e.g., mixtures of ethylene and propylene oxides.
Non-limiting examples of chain extenders include, but are not limited to, compounds having hydroxyl or amino functional group, such as glycols, amines, diols, and water. Specific non-limiting examples of chain extenders include ethylene glycol, diethylene glycol, propylene glycol, dipropylene glycol, 1,3-propanediol, 1,4-butanediol, 1,3-butanediol, 1,5-pentanediol, neopentyl glycol, 1,6-hexanediol, 1,9-nonanediol, 1,10-decanediol, 1,12-dodecanediol, ethoxylated hydroquinone, 1,4-cyclohexanediol, N-methylethanolamine, N-methylisopropanolamine, 4-aminocyclohexanol, 1,2-diaminoethane, 2,4-toluenediamine, or any mixture thereof. At times, the chain extenders are 1,3-propanediol, 1,6-hexanediol, 1,4-butanediol, or 1,9-nonanediol.
As needed for the TPEU synthesis, a suitable solvent may be used. Commonly used solvents may be chosen from the group including but not limited to aliphatic hydrocarbons (e.g., hexane and cyclohexane), organic esters (e.g., ethyl acetate), aromatic hydrocarbons (e.g., benzene and toluene), ethers (e.g., dioxane, tetrahydrofuran, ethyl ether, tert-butyl methyl ether), halogenated hydrocarbons (e.g., dicholoromethane and chloroform), and other solvents (e.g., N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO)).
Also as needed for the TPEU synthesis, a suitable catalyst may be used. The catalyst component may include tertiary amines, organometallic derivatives or salts of, bismuth, tin, iron, antimony, cobalt, thorium, aluminum, zinc, nickel, cerium, molybdenum, vanadium, copper, manganese and zirconium, metal hydroxides and metal carboxylates. Tertiary amines may include, but are not limited to, triethylamine, triethylenediamine, N,N,N′,N′-tetramethylethylenediamine, N,N,N′,N′-tetraethylethylenediamine, N-methylmorpholine, N-ethylmorpholine, N,N,N′,N′-tetramethylguanidine, N,N,N′,N′-tetramethyl-1,3-butanediamine, N,N-dimethylethanolamine, N,N-diethylethanolamine. Suitable organometallic derivatives include di-n-butyl tin bis(mercaptoacetic acid isooctyl ester), dimethyl tin dilaurate, dibutyl tin dilaurate, dibutyl tin sulfide, stannous octoate, lead octoate, and ferric acetylacetonate. Metal hydroxides may include sodium hydroxide and metal carboxylates may include potassium acetate, sodium acetate or potassium 2-ethyl hexanoate.
DESMOPHEN® 2000 (molecular weight 2000 g/mol), the petroleum based poly(ethylene adipate) diol (PEAD) used was procured from Bayer Materials Science, Canada. 1,7-heptamethylene diisocyanate (HPMDI) was synthesized according to a previously reported procedure. The petroleum-based stannous octoate (Sn(Oct)2) catalyst, 1,4-butanediol (BD), 1,6-hexanediol (HD), 1,9-nonanediol (ND) and the 1, 3-propanediol (PD) were purchased from Sigma Aldrich, Canada. All these four diols, namely, BD, HD, ND, and PD, are also obtainable from bio-based sources. Chloroform, methanol, and DMF were obtained from ACP chemical Int. (Montreal, Quebec, Canada). All reagents except DMF was used as obtained. DMF was purified by drying overnight using 4A molecular sieves followed by a vacuum distillation (˜20 mm Hg).
A series of HPMDI based TPEUs were prepared by reaction of poly(ethylene adipate) diol (PEAD) and/or aliphatic diol chain extenders (PD, BD, HD and ND) with bio-based diisocyanate, HPMDI, by using the industrially used one-shot (Method 1 and 2), pre-polymer (Method 3 and 4) and the multi stage polyaddition (Method 5) polymerization methods. The NCO:OH ratio for all TPEU samples was fixed at 1.1:1.
Table 1 provides the nomenclature and the chemical composition of the TPEUs. The samples were labelled based on the chemical composition of the repeating units represented as [CmI]x-[P(CmI)y]z, where [CmI]x is the hard segment block (HS-block) with x number of repeating HPMDI-chain extender units. The soft segment block [P(CmI)y]z included polyester diol (P=2000 g/mol) linked to either HPMDI (I) (y=1 when (CmI)y=0) or (CmI)y units and had a length given by z number of repeating units. TPEUs were designated according to the following structure:
A schematic representation of the TPEU repeating unit structure is shown in
An excess amount of HPMDI (5.5 mmol) was dissolved initially in 16 mL of anhydrous DMF under a N2 atmosphere in a three-neck flask, and stirred. In Method 1, the 1,9-nonanediol and Sn(Oct)2 dissolved in anhydrous DMF (20 mg/5 mL) was added through an addition funnel fitted to the three-neck flask. The reaction mixture was then stirred at 80° C. for 3 h (act 1). The 1,9-nonanediol was substituted by PEAD in Method 2 (Table 1) and reacted at 85° C. for 4 h. Schematics of the reaction are given in Scheme 1. The reaction mixtures were precipitated into a large excess of warm distilled water (˜50° C.). The solid obtained was filtered and dried before purification by dissolving in CHCl3 (1 g/10 mL) and a subsequent precipitation using excess methanol (methanol/chloroform=10:1). The powder obtained was dried and melt pressed at 150° C. to make films at a controlled cooling rate of 5° C./min on a Carver 12 ton hydraulic heated bench press (Model 3851-0, Wabash, Ind., USA).
In the pre-polymer method, varying ratios of HPMDI and 1,9-nonanediol (Method 3) (Table 2) was reacted according to act 1 of the previous method to prepare aliphatic diol-HPMDI pre-polymer mixtures. PEAD and catalyst dissolved in anhydrous DMF were introduced into the pre-polymer mixture in act 2 and reacted at 85° C. for another 20 h. Methods 3 and 4 differed only in the sequence of addition of the PEAD and 1,9-nonanediol reacting species. The 1,9-nonanediol reagent for act 1 reaction is replaced with PEAD in Method 4. Consequently, in act 2, the PEAD-HPMDI pre-polymers obtained were reacted with 1,9-nonanediol in the presence of catalyst at 85° C. for 20 h. The reaction schemes for Methods 3 and 4 polymerization are provided in Scheme 2. The reaction mixtures were purified and molded into films following the same procedure as in the previous method.
In the multi-stage polyaddition method, a small fraction (Table 2) of chain extender solution (1 g/10 mL in anhydrous DMF) containing Sn(Oct)2 catalyst (20 mg/5 mL anhydrous DMF) was first added to HPMDI solution taken in a three neck flask under a N2 atmosphere, and stirred. The reaction mixture was heated to 80° C. and reacted for 3 h to obtain chain extended HPMDI pre-polymers (act 1). In act 2, PEAD solution (1 g/10 mL in anhydrous DMF) containing Sn(Oct)2 catalyst (20 mg/5 mL anhydrous DMF) was added (Scheme 3), and the temperature was raised to 85° C. The reaction was continued for another 4 h. In act 3, the remaining fraction of the chain extender solution (1 g/10 mL in anhydrous DMF) containing Sn(Oct)2 catalyst (20 mg/5 mL anhydrous DMF) was added and reacted for another 16 h. The product was purified and molded into films following the previously stated procedure.
Analytical Characterization Techniques of TPEUs 1H-NMR was used to analyze the pre-polymers and the final TPEU polymers. The spectra were recorded on a Bruker Advance III 400 spectrometer (Bruker BioSpin MRI GmbH, Karlsruhe, Germany) at a frequency of 400 MHz, using a 5-mm BBO probe, and were acquired at 25° C. over a 16-ppm spectral window with a 1-s recycle delay, and 32 transients. NMR spectra were Fourier transformed, phase corrected, and baseline corrected. Window functions were not applied prior to Fourier transformation. Chemical shifts were referenced relative to residual solvent peaks.
Gel Permeation Chromatography (GPC) was used to determine the number average molecular weight (Mn), weight-average molecular weight (Mw) and polydispersity index (the distribution of molecular mass, PDI=Mw/Mn) of TPEUs. GPC tests were carried out on a Waters Alliance (Milford, Mass., USA) e2695 separation module (Milford, Mass., USA), equipped with Waters 2414 refractive index detector and a Styragel HR5E column (5 μm). Chloroform was used as eluent with a flow rate of 0.5 mL/min. The sample was made with a concentration of 2 mg/mL, and the injection volume was 30 μl for each sample. Polystyrene Standards (PS, #140) were used to calibrate the curve.
Calorimetric studies of TPEUs were performed on a DSC Q200 (TA instrument, Newcastle, Del., USA) following the ASTM D3418 standard procedure under a dry nitrogen gas atmosphere. The sample (5.0-6.0 mg) was first heated to 180° C., and held at that temperature for 5 min to erase the thermal history; then cooled down to −90° C. with a cooling rate of 3° C./min. The sample was heated again (referred to as the second heating cycle) with a constant heating rate of 3° C./min from −90° C. to 180° C.
Thermogravimetric Analysis was carried out using a TGA Q500 (TA instrument, Newcastle, Del., USA.) following the ASTM E2550-11 standard procedure. Samples of −10 mg were heated from room temperature to 600° C. under dry nitrogen at constant heating rates of 10° C./min.
The static mechanical properties of the synthesized polymer films were determined at room temperature (RT=25° C.) by uniaxial tensile testing using a Texture Analyzer (Texture Technologies Corp, NJ, USA) following the ASTM D882 procedure. The sample was stretched at a rate of 5 mm/min from a gauge of 35 mm.
The crystalline structure of selected TPEUs was examined by wide-angle X-ray diffraction (WAXD) on an EMPYREAN diffractometer system (PANanalytical, The Netherlands) equipped with a filtered Cu—Kα radiation source (λ=1.540598 Å), and a PIXcel3D area detector. TPEU samples were crystallized from the melt at a controlled cooling rate of 5° C./min. The scanning range was from 3.3° to 35° (28) with a step size of 0.013°; 2414 points were collected in this process. The deconvolution of the spectra and data analysis was performed using PANanalytical's X′Pert HighScore 3.0.4 software. For weakly crystalline TPEUs the diffraction peaks characteristic of the crystalline phase was superimposed on a broad halo indicative of the presence of an amorphous phase. The amorphous contribution to the WAXD pattern was fitted with a linear combination of two lines (centered at 4.0 and 4.7 Å) as customarily done for semi crystalline polymers.
As referenced previously, three series of high molecular weight TPEUs were prepared by reacting bio-based diisocyanate (HPMDI) with aliphatic diols (Cm) and a PEAD macro diol (2000 g/mol), by utilizing five different polymerization methods (Schemes 1-3, Table 2). Table 3 details the composition, molecular weight and the renewable carbon content (RCC, wt %) obtained for these multi block polymers. TPEUs in S1 series have varying HS-block content (0-100 wt %) ([CmI]x-[P(CmI)y]z: x and z varies while m and y are constant) with a fixed PEAD chain length (2000 g/mol).
The S2 series TPEUs have a same gross composition, e.g., a fixed HS-block content (46 and 16 wt %), but a varying distribution of HS block ([CmI]x) units. TPEUs belonging to S3 series have fixed HS-block content as well as distribution, but with a variation in the methylene chain lengths of the chain extender (Cm) units.
The composition of TPEUs was estimated from 1H-NMR using the relative intensities of the proton peaks arising from PEAD macro diol and the aliphatic diol (Cm, m=3, 4, 6, 9) units.
For S2 and S3 series of TPEUs, the sequence distribution (x:y) of HS-blocks was also determined by 1H-NMR analysis. Aliquots of the (CmI)y hard segment pre-polymer samples obtained after act 1 were end-capped by reacting with dibutyl amine, and analyzed by 1H NMR (
For the S3 series of TPEUs, with decreasing m, the proton peaks due to —NHC(═O)O—CH2— was observed at lower magnetic fields. This is due to the deferent effect of the electron-withdrawing effects by the urethane groups on the CH2 moieties. In Table 3, the PEAD: Cm molar ratio obtained from 1H-NMR analysis for S3 series of TPEUs decreased with CH2 chain length (m) due probably to some trans-esterification reaction between the chain extender (Cm) methylene units and (CH2)2 unit of PEAD. The chemical shift of the transesterification product is overlapped at δ=4.26-4.24 ppm.
The weight average molar mass (Mw) and polydispersity index (PDI) of the TPEUs determined by GPC are also listed in Table 3. The TPEU chains were sufficiently long so that they had little effect on the physical properties, and the size, distribution and composition of the block segments determined the macroscopic properties. Samples had good solubility in DMF and chloroform. The poor solubility of PU1-100x-0y0/z0-9 in chloroform at room temperature (RT=25° C.) restricted its molar mass as determination by GPC. The large chain length and low PDI values for TPEU suggested high reactivity of bio-based HPMDI towards polyaddition reactions.
The pure HS-block TPEU (PU1-100x-0y0/z0-9) exhibited two thermal transition regions during the second heating. The glass transition of amorphous [Cm=9I]x units of HPMDI-ND chains appeared at 4.5±0.5° C. (Tg1, Table 4) and the melting transition, Tm1, peaked at 124.1±0.2° C. (enthalpy of 71±0.4 J/g, Table 4). The pure SS-block TPEU (PU2-0x0-92y0/z74) exhibited a glass transition (Tg2=−38.5±0.1° C.) and a sharp melting by PEAD (P) units (Tm2=38.5° C., ΔHm2=49 J/g, Table 4) at relatively lower temperatures than HS-blocks. The melting point as well as the crystallinity of PEAD segment in TPEUs, as reflected by the enthalpy values was much lower than pure PEAD, which suggests that the crystallites are relatively less stable and less organized than in the pure PEAD macro diol. An estimation of the degree of HS-block crystallinity for HPMDI based TPEUs was restricted by the lack of fusion enthalpy data for 100% crystalline HPMDI-ND systems. The pure HS-block TPEU is a unique aliphatic m, n polyurethane [O—(CH2)m—OC(O)—NH—(CH2)n—NH—C(O)] where m=9 and n=7 represent the uninterrupted methylene groups originating from the Cm=9 diol and HPMDI (n=7). The PU1-100x-0y0/z0-9 melt transition data is however consistent with those obtained for its closest analogues, namely, the 8, 6 aliphatic polyurethane (162° C. and 60 J/g) and 10, 6 polyurethane (161° C. and 51 J/g).
The SS-block glass transition of TPEUs, as shown in Table 4, was only slightly larger than pure PEAD (Tg2=−38.5±0.1° C.) and was also independent of the HS-block content (24-92%, S1 series), distribution (S2 series) and type (Cm: m=3, 4, 6, 9-S3 series), indicating a relatively small amount of hard segments mixing with the amorphous PEAD segments. The slightly higher value obtained for Tg2 compared to pure PEAD arose from the restrictions placed at the PEAD soft segment chain ends by the covalently linked HS-blocks. No separate HS-block Tg was detected, which may be the case reported for segmented TPEUs even though an amorphous phase of HS-blocks normally exists for these types of TPEUs.
The data in Table 4 indicate that the crystallinity of both the HS- and SS-blocks was impacted by the content (S1 series), distribution (S2 series), and type (S3 series) of HS-block units. For series 1 TPEUs, the low HS-content (3 wt %) inhibited the crystallization of HS-blocks in PU4-3x1-88y0/z3-9 and resulted in amorphous HS domains. The HS-block melting temperature (Tm1) increased with increasing number of repeating HS-block units (x=1-5; HS content=16-74 wt %) and approached that of PU1-100x-0y0/z0-9 having the same composition as the repeating HS-block unit. The fusion enthalpies, reflecting the degree of crystallinity, also increased with x. Since DSC indicated minimal miscibility between HS- and SS-blocks, the well-known Flory's correlation between HS melting point and size (x) was tested for 51 polyurethanes.
where Tm is the melting point, R the gas constant, x the number of repeat units,
The development of SS-block melting transition for S1 series TPEUs was also investigated. As seen from Table 4, the lower PEAD content TPEUs (24-40%) did not exhibit any thermal transition indicative of crystalline ordering within the SS-blocks. This suggested that crystallization of PEAD units with fixed length (2000 g/mol) was limited due possibly to a confinement effect by the strongly crystallizing HS-blocks. PEAD crystallinity, however, was observed in TPEUs with a higher PEAD content (>49%). The PEAD melting temperature and enthalpy varied with HS-block content (Table 4). Interestingly, for TPEUs with intermediate PEAD contents (e.g., 49-76%), both HS- and SS-blocks were capable of crystallization and the SS-block melting temperature varied between room temperature (RT=25° C.) and the melting temperature of pure SS-block TPEU.
The PEAD confinement by HS-blocks was further investigated for S2 series TPEUs having a fixed HS-block content and PEAD chain lengths, but with varying distribution of HS block lengths (x, y). For TPEUs with 46% HS-block content (PU3-46x3-49y0/z1-9), the PEAD melting temperature and enthalpy increased with increasing distribution of HS-blocks ([Cm=9I]x with x varying from 3 and 0-3). In PU4-46x0-3-49y0/z1-9 sample with a broad distribution of HS-blocks, (Cm=9I)x=3, the hard segment blocks crystallized to a high level of ordering and restricted the space available for the crystallization of the PEAD chains, thereby decreasing Tm2 and enthalpy values (Table 4). This finding was of significant technical importance as one can control the crystallization of soft segments by controlling the dispersion of hard segment blocks in semi-crystalline PEUs.
The crystal structures of the TPEUs were analyzed using WAXD.
In order to show the crystalline peaks more prominently and reveal the phase type of PU1-100x-0y0/z0-9, PU3-46x3-49y0/z1-9 and PU4-46x0-3-49y0/z1-9, the background and the amorphous contribution were subtracted from their WAXD patterns and presented in
The WAXD pattern for PU1-100x-0y0/z0-9 presented diffraction peaks at 4.41 Å, 3.82 Å and 3.63 Å attributable to (100), (010) and (110) reflections of a monoclinic subcell. In this crystal structure, the HPMDI-ND (Cm=9I)x chain segments form planar sheets in order to maximize the contribution of the C═O . . . H—N hydrogen bonds between adjacent chains. The WAXD pattern for both PU4-46x0-3-49y0/z1-9, and PU3-46x3-49y0/z1-9 displayed the (100), (010), and (200) reflections of the monoclinic symmetry and (110) and (200) reflections of an orthorhombic subcell. The intensity of the peaks originating from the monoclinic phase due to HS-blocks were much higher than the weak peaks of the PEAD orthorhombic phase. Another very weak scattering peak was observed in the WAXD pattern of PU4-46x0-3-49y0/z1-9 at 4.6 Å attributable to the characteristic (100) reflection of a triclinic phase, labeled T. The high level of HS-block ordering was also evident from the unchanged melting temperature and enthalpy values for the HS-block in these TPEUs. A distribution of hard blocks in PU4-46x0-3-49y0/z1-9 sample crystallizes into identical close packing (comparable) but imposes constraints to crystallization of PEAD soft blocks and pushes the PEAD melting down further to lower temperatures.
The HS- and SS-block crystallization for PU5-46x0-2-49y1/z1-m samples as a function of the chain extender methylene chain length (Cm where m=3, 4, 6, 9: S3 series) was also investigated. An odd-even effect on HS-block melting temperature (Tm1) was observed for S3 series of TPEUs (
The PU5-46x0-2-49y1/z1-4 sample with 1,4-butanediol chain extended HPMDI hard block units gave the highest melting temperature (Tm1=142.9±0.6° C.), which is explained by the unique conformations adopted by even numbered (m=4) methylene chains to maximize the urethane-urethane H-bonding. Interestingly, the PEAD melting (Tm2) was affected by the HS-block odd-even effects. As seen from
Mechanical performance of the HPMDI based TPEUs were evaluated by measuring the initial modulus, tensile strength and extensibility, and was further compared with petroleum-based TPEUs prepared from PEAD and butanediol chain extender and petroleum based diisocyanates, as outlined in Table 6.
The pure HS-block polymer, PU1-100x-0y0/z0-9 was too brittle to make tensile specimens. The S1 series TPEUs demonstrated deformation behavior ranging from that of a plastic (ductile) to one of an elastomer (rubber-like) depending on the HS-block content. For TPEUs with higher HS-block content (>49 wt %) the stress-strain curves showed plastic failure with limited extensibility (% EB of 6-80%,
The low HS-content TPEUs, PU4-3x1-88y0/z3-9 and the pure SS-block PU2-0x0-92y0/z74, which lack crystallization by HS-blocks (refer DSC data, Table 4) but have crystallized SS-blocks instead, exhibited enhanced tensile strength (
It is notable that the SS-block crystallites play a significant role in the mechanical performance of TPEUs. For semi-crystalline TPEUs with constant HS block content (S2 series) the tensile strength and extensibility increased with increasing distribution (x) of the HS-blocks (Table 6). This notably contradicts the behavior of classical segmented TPEUs where monodisperse HS-blocks were shown to offer higher tensile strength and modulus, due to a better phase separation and close packing. The PU3-46x3-49y0/z1 sample, for example, deformed plastically whereas PU3-46x3-49y0/z1, with the same HS-block content but x varying from 0 to 3, is an elastomer (Table 6). This was clearly a product of the latter possessing SS-block crystallites with a room temperature melting transition (DSC data, Table 4). The SS-block crystallites reinforce the polymer matrix at temperatures below their melting transition. The SS-block crystallites with a room temperature melting transition undergo reversible matrix reinforcements during deformation due to soft segment chain mobility that allows for the newly formed junctions to serve as load bearing phases and thereby improve the toughness.
For S3 series TPEUs having fixed HS-block content (46 wt %) and distribution (x=0-2, y=1) but vary only in their chain extender lengths (Cm, m=3, 4, 6, 9), the mechanical properties strongly resembled their HS-block odd-even melting behavior. As seen in
The thermal stability of TPEUs was investigated using TGA analysis at a heating rate of 10° C./min. Example DTG curves obtained for the S1 series are shown in
PU1-100x-0y0/z0-9 exhibited a two-act degradation process. The decomposition of urethane bonds started at a temperature above 200° C. (250.5±0.8° C.), similarly to m, n aliphatic polyurethanes with high H-bond densities. Decomposition reaches its maximum rate at 290-301° C. (Td1 at 290.5±0.4 with a shoulder at Td2=301.9±0.3° C.) accompanied by a major weight loss, ΔW1. The more stable urethane structures in pure HS-block TPEU underwent decomposition during the second degradation stage (Td3=454.2±0.8° C., ΔW2=19±1.0%).
A higher initial decomposition temperature was recorded for S1 series TPEUs with PEAD SS-block contents as is shown in
The decomposition temperatures for TPEUs derived from bio based HPMDI were not affected by the preparation methods and are comparable to the thermal stability temperatures (250-300° C.) reported for similar systems based on hexamethylene diisocyanate (HDI), the closest petroleum based analogue of HPMDI. Moreover, these materials can be processed by injection molding and extrusion since their thermal stabilities are well above the optimum thermoplastic processing window.
To review, high molecular weight thermoplastic polyester urethanes (TPEUs), [CmI]x-[P(CmI)y]z with crystallizable hard ([CmI]x, HS-block) and soft blocks ([P(CmI)y]z, SS-block) were prepared from vegetable oil-based HPMDI (I), PEAD macro diol (2000 g/mol) (P), and aliphatic diol chain extenders (Cm, m=3, 4, 6, 9) using one-shot, pre-polymer and multistage polyaddition methods. For fixed PEAD chain lengths (2000 g/mol) the relative roles of hard and soft segment thermal transitions on the mechanical performance was examined for varying content (x, y—series S1), distribution (x, y, z—series S2) and types (Cm, m=3, 4, 6, 9—series S3) of HS-block units in TPEUs. The HS-blocks including HPMDI-Cm=9 units crystallized freely into monoclinic crystal packing, whereas the crystallization of PEAD segments into orthorhombic symmetry was constrained by the HS-block ordering for TPEUs. For TPEUs with a fixed HS-block content (46 wt %), the SS-block melting temperature and enthalpies were lowered with increasing HS-block distribution, as well as by chain extenders with even numbered methylene groups (m=4, 6).
The semi-crystalline thermoplastic polyester urethane elastomers prepared from bio-based heptamethylene diisocyanate possess toughness and strength comparable to those made from petroleum-based diisocyanates. These TPEUs are thermally stable up to 250° C. A significant reinforcement effect due to PEAD crystallites mitigate the lowering of modulus and strength for elastomeric TPEUs at lower HS-block contents (<46 wt %). For TPEUs with fixed HS-block content (46 wt %, S2 series), the presence of SS-block crystallites imparted elastomeric properties to an otherwise thermoplastic TPEU. This study demonstrates that the control of hard segment crystallization has the potential for tailoring the soft segment crystalline behavior in TPEUs to achieve tunable mechanical properties.
The foregoing detailed description and accompanying figures have been provided by way of explanation and illustration, and are not intended to limit the scope of the invention. Many variations in the present embodiments illustrated herein will be apparent to one of ordinary skill in the art, and remain within the scope of the invention and their equivalents.
A claim of priority for this application under 35 U.S.C. §119(e) is hereby made to U.S. Provisional Patent Application No. 62/051,821 filed Sep. 17, 2014; and this application is incorporated herein by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 62051821 | Sep 2014 | US |
