Effect of Soft Segment Polydispersity on the Elasticity of Polyurethane

Jan 31, 2013 - A family of copolyurethane elastomers was synthesized, based on 1,4-butanediol (BDO) or diethylene glycol (DEG) and hard segments of 4,...
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Effect of Soft Segment Polydispersity on the Elasticity of Polyurethane Elastomers Cristina Prisacariu,* Elena Scortanu, Sergiu Coseri, and Bogdan Agapie Institute of Macromolecular Chemistry “Petru Poni”, Aleea Grigore Ghica Voda nr. 41A, Iasi, Romania S Supporting Information *

ABSTRACT: A family of copolyurethane elastomers was synthesized, based on 1,4-butanediol (BDO) or diethylene glycol (DEG) and hard segments of 4,4′-dibenzyl diisocyanate (DBDI) of constant uniform length. The materials had comparable molecular weights. The soft-segment (SS) polydispersity was varied. A broad polydispersity was achieved by blending polyols of different molecular weights of poly(oxytetramethylene) (PTMO650, PTMO1000, and PTMO2000). To obtain a narrow soft segment molecular weight distribution, fractionated samples of PTMO1546 were prepared. The tensile and thermomechanical properties were determined. Results were related to microstructural changes, on the basis of evidence from wide-angle X-ray scattering (WAXS). Increasing the SS polydispersity from 1.1 to 1.72 resulted in an improvement of the mechanical properties. Inelastic effects were most pronounced when the hard segment crystallized and in the case of the materials achieved with a narrow SS polydispersity.

1. INTRODUCTION The block co-polyurethanes (PUs), consisting of isocyanate hard segments, macrodiol soft segments, and chain extenders, represent an extraordinarily versatile family of polymers, where a wide variety of physical properties may be achieved via variations in chemical composition and synthesis route. Two important structural features that influence the physical, mechanical and thermal properties of the PUs are the length and chemical structure of the hard segment (HS) and soft segment (SS). The HS and SS molecular weight distribution, particularly with higher HS concentrations, influence the degree of microphase separation, crystallinity, and domain size of the PUs.1−3 An increase in length of the SS and HS molecular weight has been observed to determine the enhance in the PUs microphase separation degree where more-developed reinforcing hard domains structure have been observed. As shown by Harrell,4 a decrease in the HS sequence molecular weight distribution leads to a significant increase of modulus, tensile strength, and elongation.5,6 The choice of polyol and its molecular weight greatly affect its physical state and the elastomeric properties of the PUs.7−9 Important characteristics of polyols are their molecular weight and polydispersity, percentage of primary hydroxyl groups, functionality, and viscosity. A well-known method to influence the PUs thermal and mechanical behavior is by controlling the molecular weight distribution of the HS and SS. Reducing the SS polydispersity was observed to result only in minor improvements of the modulus, tensile strength, and elongation at break of the PUs. Lunardon et al. investigated a series of PUs based on poly(ε-caprolactone) and 1,4-butanediol (BDO) with constant uniform HS lengths.5 To optimize the thermal and mechanical properties, the SS polydispersity was varied between 1.09 and 1.87. The PUs had comparable molecular weights. The variation in the thermal and mechanical © 2013 American Chemical Society

properties was attributed to the differences in the SS polydispersity. Decreasing the SS polydispersity resulted in an increase in the HS melting enthalpy, accompanied by a moderate increase in the tensile properties. Ng et al.10 described the synthesis and properties of block copoly(ether urethane)s, in which the urethane part was based on piperazine that was unable to form hydrogen bonds, which made the interpretation of the PUs structure−property relationships easier. The authors observed only some minor effects (i.e., moderate increases in the tensile strength and elongation at break) upon lowering the polydispersity of the soft sequence. More recently, Pechold and Pruckmayr studied the effect of the SS molecular weight distribution on PUs based on polyester macrodiols. Increasing the polydispersity for materials obtained with SS molecular weights of 1000 g/mol determined the improvement of the tensile properties; however, for PUs with higher SS molecular weights, this effect was not observed and the influence on the tensile strength was unclear.11 In addition, as shown by Lunardon et al.,5 lowering the SS polydispersity of polyester-based PUs derived from methylene diphenyl methane diisocyanate (MDI), BDO, and linear random copolyesters from adipic acid resulted in an increase in the permanent set of PUs. Therefore, the influence of the different chemical compositions used still makes the interpretation difficult. Therefore, although various parameters have been considered in different studies, the outcome is still not clear; sometimes, the results are still contradictory, so a better understanding of the effects of the HS and SS polydispersity is still needed. Received: Revised: Accepted: Published: 2316

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The PUs with a more regular structure given by an increased HS ordering were observed to exhibit a more rigid network which resulted in higher strain energies but lower tensile strength along with higher residual elongation. An enhanced HS ordering leads to a moderate decrease of the performance of PUs as elastomers. In the present work, our aim has been to help explain the characteristic features of the mechanical response of a family of DBDI-based PUs with a constant HS length, by varying the SS molecular weight distribution. Results are drawn on a series of materials in which the chemical structure is known, and the physical structure has been well-characterized.

In recent years, we have made a series of studies of different aspects of the mechanical behavior of PUs, aimed at improving our understanding of the way the macroscopic properties of these materials are dependent on chemical structure and nanometer-scale physical structure. The materials were all synthesized in the Romanian laboratory. The range of structures achieved was widened beyond that normally available, by inclusion of a diisocyanate with unusually large conformational mobility (4,4′-dibenzyl diisocyanate (DBDI)). A systematic investigation has been performed using the structural features of the PUs (phase segregation and degree of crystallinity). The use of this diisocyanate allows the specific −Ph−CH2−CH2−Ph− moiety to introduce a variable geometry into the HS due to the possibility of internal rotation of this isocyanate around the −CH2−CH2− bridge between the phenyl rings.3,12−15 It is notable that, as we previously showed, the DBDI-based polymers give rise to significant degrees of crystallinity, in the presence of suitable chain extenders, because it is the lowmolecular-weight ethylene glycol (EG) or BDO.3,13,14 The relative ease of crystallization in DBDI, compared to that observed in conventional diisocyanates, is readily explained in terms of greater flexibility of the DBDI molecule, arising from its −(CH2)2−. In contrast with the conventional PUs based on rigid HS such as 4,4′-methylene bis(phenyl isocyanate) (MDI), the DBDI hard segments can adopt a linear conformation facilitating packing (Figure 1) and interchain hydrogen

2. EXPERIMENTAL SECTION 2.1. Materials. The model PUs synthesized for this work were all three-component systems (see Table 1), combined in stoichiometric proportions, and consisting of (1) a diisocyanate (DI), such as DBDI; (2) a macrodiol (MD), such as PTMO; and (3) a small molecule diol as a chain extender (CE), such as anhydrous diethylene glycol (DEG) and 1,4-buthylene glycol (BDO). All the materials were achieved with constant uniform HS lengths and the same HS weight fraction (followed using a gel permeation chromatography (GPC) technique). MDs used in this study were poly(oxytetramethylene) diol with various molar masses: 650 (PTMO650), 1000 (PTMO1000), and 2000 (PTMO2000), which were kindly supplied by BASF. Another PTMO with a molar mass of 1546 (PTMO1546) has been obtained by mixing PTMO1000 and PTMO2000 or PTMO650 and PTMO2000. In view of the better elastomeric properties produced, a small excess of 10% NCO groups against the total hydroxyl (OH) amount proceeded from the polyol and CE was prepared, using a deficit of CEs, as described elsewhere.13−15 The three components were always mixed in the molar proportions of DI:MD:CE = 2:1:0.81, giving HS mass fractions in the region of 40%, and an isocyanic index of I = 110. All materials described here had ∼60 wt % SS and polydispersity indices (PDI) between 1.1 and 1.72 (see Table 2). The synthesis was complete only when there was total consumption of isocyanate excess by ambient humidity.15 Synthesis was carried out by the prepolymer route, as described previously by Prisacariu et al.13−15 The final result was a polymer with a molecular weight (Mw) in the range of 60−120 kg mol−1, in the form of sheets with a thickness in the range of 0.3−0.6 mm. The sheets were stored at room temperature for at least one month before testing. By blending polyols of different molecular weights (i.e., PTMO650, PTMO1000 and PTMO2000), a broad polydispersity polyol was obtained with a similar molecular weight. Thus, in order to increase the SS molecular weight distribution, we have also prepared PTMO1000 by mixing PTMO650 and PTMO2000 in corresponding proportions. To reduce the polydispersity, i.e., to obtain a narrow SS molecular weight distribution, fractionated

Figure 1. 4,4′-dibenzyldiisocyanate (DBDI) extended form.

bonding. As compared to the DBDI-based hard sequence, the MDI hard segments are intrinsically kinked in shape, reducing conformational mobility and thereby hindering close packing and the achievement of hydrogen bonding.3 In one of our recent works, for selected systems of PUs with HS of either MDI (noncrystallizing) or DBDI (crystallizing), we have studied the influence of the HS polydispersity and the effect of the HS ordering the mechanical behavior of PUs.8 An inverse step-by-step synthesis was carried out that started with the construction of the hard-ordered reactive intermediate, followed by the reaction with the macrodiol, resulting in an increased hard sequence ordering.8 The effects of the preparation method and that of the HS ordering on the elastomeric performance of the PUs were followed by means of tensile tests and thermomechanical analysis.

Table 1. Compositions, SS Molecular Weight, and Size Distribution of the Family of Polyurethane Elastomers PU1−PU10 Prepared and Studied in This Work type of CE

PTMO1000, standard

PTMO1000,a enhanced SS polydispersity

PTMO1546,b enhanced SS polydispersity

PTMO1546,c broad SS polydispersity

PTHF1546 fraction, narrow SS polydispersity

BDO DEG

PU1 PU2

PU3 PU4

PU5 PU6

PU7 PU8

PU9 PU10

a

Composed of 52% PTHF2000 + 48% PTHF650. bComposed of 70.6% PTHF2000 + 29.4% PTHF1000. cComposed of 85.86% PTHF2000 + 14.14% PTHF650. 2317

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3. RESULTS AND DISCUSSION 3.1. Structural Studies: Crystallinity Measurements. The WAXS data indicated a general two-phase structure for the PU systems. Figure 2 shows examples of 1-D WAXS patterns for material PU5 extended with BDO (crystallizing) and material PU6 extended with DEG (noncrystallizing).

samples of PTMO were prepared. The fractionated samples were achieved by multiple extractions with selected solvents removing, thus, the more-soluble fractions (with lower weights) in methanol. An increase in the weight of the insoluble fraction from 1000 to 1546 was obtained, which can only be achieved by decreasing the polydispersity degree. Molecular weights were measured via gel permeation chromatography (GPC), as previously described,16 on 0.5% polymer solutions in NMP, using dimethyl formamide (DMF) as an eluent and an evaporative light scattering detector (Model PL-EMD 950) (polystyrene calibration). Table 1 shows the composition family of polyurethane elastomers PU1−PU10 prepared and studied in this work. Also shown in the Supporting Information section is Table S1, which includes the GPC analyses performed for the series of polyurethane elastomers prepared and studied in this work (PU1−PU10). 2.2. Structural Studies. Information on the microstructures of the materials was obtained from WAXS experiments using Cu Kα radiation and also using the synchrotron radiation facilities at the Daresbury Laboratory, U.K., as reported elsewhere.3,14 In the materials extended with BDO and DBDI as the hard segment, sharp peaks were observed in the WAXS intensity versus 2θ scans, indicating phase separation and crystallization in the hard domains. The scattering was separated into amorphous halo (Ia) and crystal diffraction peak (Ic) components by fitting Gaussian peaks to those crystal diffraction peaks visible, following the procedure used in a previous paper,3 and a degree of crystallinity (χ) was determined:3 crystallinity index =

Figure 2. Example one-dimensional (1-D) WAXS patterns for the DBDI/BDO-based material PU5 and the DBDI/DEG-based material PU6, both of them obtained with an enhanced SS polydispersity (PTMO1546 = 70.6% PTHF2000 + 29.4% PTHF1000).

As seen in Figure 2 and also in Table 2 below, the only polymers to have more significant crystallinity are all those based on the couple BDO−DBDI. This is consistent with previous reports of comparisons between melt-processed polyether and polyester PUs of constant higher molecular weight (Mw = 2000 ± 50). The flexible DEG inhibits crystallization.3,14 The relative easy crystallization of the DBDI-based hard segments is due to the greater flexibility of the DBDI molecule, arising from its −(CH2)2− bridge between the phenyl rings.13,14 When DEG is used as chain extender with DBDI, the central −O− atom of DEG introduces kinks into the DBDI hard segment and disrupts the chain packing that could otherwise be achieved. Table 2 contains the values of the crystallinity index (χ) for the entire series of polymers labeled in Table 1.

∫ Ic dθ ∫ (Ia + Ic) dθ

(1)

Values of the crystallinity index (χ) are included in Table 2. 2.3. Mechanical Tests. Tensile measurements and cyclic tensile tests were carried out, cycling between a fixed strain limit and zero load, using a constant nominal strain rate with a magnitude of 0.03 s−1. This was chosen for consistency with previous studies of a larger number of DBDI-based materials.3,13,14 Test specimens were cut from films using dimensions given in ASTM Standard D1708, i.e., a dumbbellshaped specimen with a length of 53 mm between shoulders, a gauge length of 20 mm (on which the strain was measured), a width of 5.8 mm, and a thickness of 0.3−2 mm. The stress− strain data on these specimens presented here were obtained using an Instron 4204 Testing Machine or a Schopper MZ Gip Testing Machine, at room temperature (T ≈ 25 °C). The data were processed to quantify specific inelastic features in the responses of the materials: tensile and cyclic tensile responses where the unrecovered strain and hysteretic energy dissipation have been followed. The specimens were extended to a given maximum nominal strain emax, followed immediately by at least three unload−reload cycles at the same rate.3,14 2.4. Thermomechanical Experiments. The thermomechanical measurements were performed on a homemade instrument as previously reported. 13,17 The course of elongation was followed under constant load of a pretensioned sample, to start from a constant initial 50% strain amplitude. The strain then was kept constant and the variation of sample elongation was followed by increasing the temperature with constant increments of 1 °C/min.

Table 2. Results for Crystallinity Index( χ) for the Series of Materials Labeled in Table 1 material

χ

material

χ

PU1 PU2 PU3 PU4 PU5

0.116 0 0.104 0 0.148

PU6 PU7 PU8 PU9 PU10

0 0.131 0 0.167 0

For the BDO/PTMO1546-based polymers PU5 and PU7, the increase of the SS polydispersity from PDI = 1.47 to PDI = 1.68, determined a moderate decrease of the crystallinity index (χ): from 0.148 (PU5) to 0.131 (PU7). This was in contrast with the BDO-based material obtained with a narrow SS polydispersity (PU9 with PDI = 1.22) where an increase of the crystallinity degree (to χ = 0.167) is observed. Similar observations can be made for the two BDO/ PTMO1000-based materials: the increase of the SS polydispersity from PDI = 1.12 (PU1) to PDI = 1.34 (PU3), determined a moderate decrease of the crystallinity index (χ): from 0.116 (PU1) to 0.104 (PU3). For the BDO-based polymers PU5 and PU7 achieved with a higher SS molecular weight (PTMO1546), the crystallinity index 2318

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determined a higher hardness and lower residual elongations from 172% (PU5) to 140% (PU7). Higher stiffness, higher 100% and 300% moduli, and higher tensile strength values are seen for the BDO-based polymers, compared to the materials achieved with DEG. For the BDObased materials, the higher values of the stiffness and residual strain (acting as measures of the polymer inelasticity) are due to the fact that, as previously shown by us, particular diols, such as BDO, promote crystallinity with DBDI while DEG has been shown to inhibit crystallinity in DBDI.3 The values of the residual elongations quoted in Tables S2− S5 in the Supporting Information are also in agreement with the crystallinity index data listed in Table S1 in the Supporting Information. By comparing the data quoted in Tables S4−S5 and Table S1 in the Supporting Information, it can be seen that, for the BDO-based polymers, the increase of the SS polydispersity resulting in lower residual elongation values is accompanied by a decrease in the degree of hard-phase crystallinity, as revealed by the decrease of the crystallinity index (χ) values in Table S1 in the Supporting Information. For example, in the case of the BDO/PTMO1546-based polymers PU5 and PU7, the increase of the SS polydispersity, from PDI = 1.47 to PDI = 1.68, determined a moderate decrease of the crystallinity index (χ), from 0.148 (PU5) to 0.131 (PU7). This was in contrast with the BDO-based material obtained with a narrow SS polydispersity (PU9, with PDI = 1.22), where an increase of the crystallinity degree to χ = 0.167 and, concomitantly, an increase in the values or the residual elongation from 140% (PU7) to 190% (PU9) were observed. A narrow SS polydispersity resulted in a decrease of σ100 and σ300 and lower tensile strength values, while the residual elongation values increased, which was accompanied by an increase of the degree of hard-phase crystallinity, as revealed by the increase of the crystallinity index (see Table S1 in the Supporting Information), from χ = 0.131 (PU7) to χ = 0.167 (PU9). A narrow SS polydispersity determined the decrease in the elastomeric behavior of the PUs. Thus, even in the absence of the hard-phase crystallinity, i.e., for the series of polymers achieved with DEG, the highest residual elongation is observed in the case of the polymer DEG-based polymer PU10 with a narrow SS polydispersity (residual elongation = 45%), as compared to significantly lower values observed for the other DEG-based polymers with an enhanced or broad polydispersity. This is consistent with the observations made by Lunardon et al.5 on PUs with MDI and BDO, where it has been shown that lowering the SS polydispersity resulted in an increase in the permanent set of PUs. 3.2.2. Cyclic Tensile Responses of PUs. Since PUs are usually employed in products where rubberlike resilience is required, their performance under small and large cyclic deformations is of special relevance. In a third stage of the present work, the materials were cycled between an extension of 3, and zero load, for 3 cycles, as shown in Figure 3. The first cycle work input E1 was obtained as detailed elsewhere,3 by integration,

values are higher than for the BDO-based materials obtained with PTMO1000. An increase in the SS molecular weight decreases the miscibility between the hard and soft sequences, leading to the formation of larger phase regions both for the DEG and BDO materials. Lowering the SS polydispersity resulted in an increase of the crystallinity index (χ): from 0.131 (PU7) to 0.167 (PU9). A narrow SS polydispersity lead to a more uniform distribution, favoring the DBDI/BDO hard segments to adopt a linear conformation which, as previously shown,3,14 is associated with the possibility of pronounced phase separation into a domainmatrix morphology, and with a higher tendency to crystallization and self-association by hydrogen bonding. No changes in the crystallinity index values are observed in the DBDI materials derived from DEG, regardless the degree of the soft sequence molecular weight distribution. All the DEGbased polymers labeled in Table 3 are completely amorphous (χ = 0). 3.2. Structural Studies: Mechanical Measurements. 3.2.1. PU Stress−Strain Data. In a second phase of our work, we investigated the stress−strain behavior and cyclic tensile responses for the range of PUs labeled in Table 1. Tables S2−S5 in the Supporting Information give the tensile properties for the entire series of PUs from Table 1. Tables S2 and S3 give data for the DEG-based polymers with PTMO1000 and PTMO1546, respectively. Similarly, Tables S4 and S5 give data for the BDO-based polymers with PTMO1000 and PTMO1546, respectively. The quantitative measures σ100 and σ300 represent the stresses necessary to produce strain amplitudes of 100% and 300%, respectively. As seen in Tables S2 and S3 in the Supporting Information, the elastomeric behavior of the series of the DEG-based PUs with PTMO1000 improves with the increase of the SS molecular weight distribution: the enhancement of σ100 and σ300 and the tensile strength is accompanied by a decrease in the residual elongation from 20% (PU2) to 10% (PU4). Similar observations can be made with regard to the DEGbased materials with PTMO1546. A general remark is that, as expected, the values of the data for the DEG/PTMO1546 materials are higher than those of the DEG/PTMO1000 polymers. The elastomeric properties improve as the SS molecular weight increases. For example, the residual elongation decreases from 20 (PU2) to 7.5 (PU8). Increasing the SS equivalent length at constant HS weight fraction results in longer (on average) hard segments, which are thermodynamically more likely to segregate into strongly reinforcing nanodomains.5,9 As shown by Lunardon et al.,5 Ng et al.,10 and our group,3 for the range of HS volume fractions below 50% (as is the case for the polymers studied in this work), much of the space is occupied by the SS matrix within which microphaseseparated nanodomains of the hard phase are dispersed. The remarks from above are also in agreement with the observations made by Pechold et al.11 for PUs based on polyester macrodiols, where it has been shown that increasing the polydispersity for materials obtained with SS molecular weights of 1000 g/mol, resulted in an enhancement of the tensile properties. With regard to the BDO-based materials with PTMO1000 and PTMO1546, as seen in Tables S4 and S5 in the Supporting Information, the PU tensile property data also increase as the SS polydispersity and molecular weight also increase (except for the residual elongation). Increasing the SS polydispersity

E1 =

∫ s de

over the first loading up to emax. The first-cycle hysteresis (ΔE1) was calculated by integration over the entire first load−unload 2319

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elastomeric behavior of the PUs) decreased with the enhancement of the SS polydispersity from 0.522 (PU2, standard  PTMO1000) to 0.440 (PU8, broad polydispersity  PTMO1546 (85.86% PTHF2000 + 14.14% PTHF650). Note that in the case of the DEG material PU10 achieved with a narrow SS polydispersity, the relative residual strain εr* increased to 0.692. This is consistent with our previous observations regarding the enhancement of the material inelasticity with decreasing SS polydispersity. Similar observations could be made with regard to the BDObased materials. The relative residual strain εr* decreased with the enhancement of the SS polydispersity: from 0.751 for PU1, standard (PTMO1000) to 0.748 for PU7 with a broad polydispersity (PTMO1546 (85.86% PTHF 2000 + 14.14% PTHF650)). As in the case of the DEG-based materials, the relative residual strain εr* increased to 0.823 for BDO material PU9 achieved with a narrow polydispersity. Note that for all of the BDO-based materials achieved with crystallizable DBDI/BDO hard sequences, the recoverability, as revealed by the inelasticity measure (ε*r), was significantly lower than that for the analogous PUs achieved with DEG. The relative first cycle hysteresis ΔE1* was also observed to be significantly higher, showing less recoverability for the BDObased materials with crystallizable HS (up to ΔE1* = 0.949) than for the polymers obtained with DEG (maximum ΔE1* = 0.869). As seen in Tables S2−S5 in the Supporting Information, the first-cycle relative hysteresis (ΔE1*) decreased with the enhancement of the SS polydispersity for both the DEG- and BDO-based polymers. However, note that the inelasticity of the two polymers obtained with a narrow polydispersity (PU9BDO and PU10-DEG) increased significantly: for the DEGbased polymers, ΔE1* increased from 0.817 (PU8) to 0.869 (PU10), and for the BDO-based materials, ΔE1* increased from 0.914 (PU7) to 0.949 (PU9). Figure 4 shows the first-cycle relative hysteresis (ΔE1*) as a function of the degree of crystallinity (χ) for the entire series of polymers from Table 1.

sequence. The second-cycle work input (E2) was obtained by integration over the second loading, from er to emax (see Figure 3). The second-cycle hysteresis (ΔE2) was determined by integration over the entire second load−unload sequence. The relative first-cycle hysteresis was determined and expressed as

ΔE1* =

ΔE1 E1

Example curves of nominal stress versus nominal strain are shown in Figure 3 for material PU7 achieved with BDO and material PU8 extended with DEG, both of them having a broad SS polydispersity.

Figure 3. Example curves of nominal stress versus nominal strain between zero stress and a nominal strain of 3 for polymers () PU7 (BDO) and (- - -) PU8 (DEG) with a broad SS polydispersity.

Prominent differences can observed between the DBDIbased polymers derived from the two chain extenders BDO and DEG. As seen in Figure 3 and Tables S2−S5 in the Supporting Information, the hysteresis energy dissipation values, as revealed by the relative first-cycle hysteresis ΔE1*, were found to be higher for material PU7 extended with BDO than that for material PU8 obtained with DEG. This is consistent with our previous results for materials with other combinations of CEs and MDs. Inelastic effects were most pronounced when the hard segment crystallized. Also determined was the residual strain on first unloading (εr), expressed as a relative residual strain: ε εr* = r εmax In comparing the materials, it was convenient to refer only to the relative first-cycle hysteresis, ΔE1*, because of the fact that, as previously shown for a large series of PUs of various chemical structures, irreversible changes to the stress−strain response are confined essentially only to the first loading cycle. In subsequent cycles, the load−unload stress−strain curves remained almost unchanged.3,14 The inelasticity measure ΔE1* was found to reside in the range of 0.803−0.949. Tables S2−S5 in the Supporting Information show the values of the relative first-cycle hysteresis (ΔE1*) and those of the first-cycle work input (E1) for the entire series of PUs listed in Table 1. For comparison reasons, the values of the first-cycle relative residual strain (ε*r) are also listed in Tables S2−S5 in the Supporting Information. Regardless of the SS degree of polydispersity, higher recoverability is observed for the DBDI/DEG-based polymers, compared to those achieved with DBDI/BDO. For the DEGbased polymers, the residual strain on first unloading expressed as relative residual strain εr* (which is a measure of the

Figure 4. First-cycle relative hysteresis (ΔE1*) as a function of the degree of crystallinity (χ) for the series of PUs depicted in Table 1. (Legend: (○) PU1, (●) PU2, (△) PU3, (▲) PU4, (◇) PU5, (◆) PU6, (⬡) PU7, (⬣) PU8, (□) PU9, and (■) PU10.)

As seen in Figure 4, for the DBDI/BDO polymers (denoted with open symbols), the first-cycle relative hysteresis (ΔE1*) increased with the enhance of the crystallinity degree (χ). Regardless the SS polydispersity degree, significantly lower hysteresis values are observed in the case of the DEG-based materials (filled symbols in Figure 4), where crystallinity is absent. 3.3. Thermomechanical Measurements of PUs. The course of elongation was followed under constant load of a 2320

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Table 3. Thermomechanical Curves for Materials PU1-PU10 Studied in This Work

SS polydispersity, the DBDI-based polymers gave rise to significant degrees of crystallinity when extended with BDO. With DBDI, when DEG was used as a chain extender, the materials were completely amorphous, regardless of the degree of SS dispersity. The mechanical and thermal experiments clearly revealed that inelastic features in the mechanical and thermal response of the PUs studied in this work were sensitive to the SS molecular weight and polydispersity. For both the DEG and BDO series of polymers, the hardness and the tensile properties values increased with the enhancement of the SS polydispersity, while the residual elongation values were observed to decrease. Higher hardness and higher residual elongations were observed in the case of the materials extended with BDO, which promotes crystallinity with DBDI, compared to the similar DBDI polymers achieved with DEG inhibiting crystallinity. A narrow SS polydispersity led to a decrease of the PUs elastomeric behavior, as revealed by a higher crystallinity index, up to χ = 0.167 (PU9), accompanied by a much more pronounced hysteresis, and unrecovered strain. The inelasticity of the material obtained with a narrow SS polydispersity increased significantly both for the PU based on crystallizable hard segments and in the case of the amorphous polymer obtained with DEG. The increase in the SS polydispersity led to the improvement in the thermal behavior of the PUs. The temperature at which the polymer rupture occurred (TR) was higher for the materials with noncrystallizing hard segments based on DEG. In contrast, a narrow SS polydispersity resulted in a decrease of the thermal properties data of the PUs, for both the DEG- and BDO-based materials. These patterns give new insight into the physical origin the elastic and inelastic effects in PUs.

pretensioned sample, to start from a constant initial 50% strain level. Shown in Table 3 are the deformation−temperature (ε− t) curves for the three materials. The temperature at which the polymer broke (TR) was considered. (See Table 3.) In principle, a better thermal behavior was observed with the increase of the SS polydispersity. The temperature at which polymer rupture occurred (TR) was higher for the DEG-based materials based on noncrystallizing hard segments than for the analogous PUs achieved with crystallizing hard segments based on the couple DBDI/BDO. A narrow SS polydispersity resulted in a reduction in the thermal properties of the PUs for both the DEG- and BDO-based materials: for PU10 (DEG), TR = 85° and for PU9 (BDO), TR = 80°. As seen, the lowest TR value was observed in the case of the polymer PU9 (BDO) with crystallizing hard segments.

4. CONCLUSIONS The present work is part of a larger ongoing study where we review and bring new contributions in understanding the relationship between the polyurethane (PU) molecular/supramolecular architecture at the nanometer scale and their macroscopic mechanical behavior. In the present work, a family of co-polyurethane elastomers of comparable molecular weight was obtained, chain extended with 1,4-butanediol (BDO) or diethylene glycol (DEG), and based on hard segments of constant uniform length derived from the flexible diisocyanate, 4,4’-dibenzyl diisocyanate (DBDI). The soft-segment (SS) polydispersity was varied from 1.1 to 1.72. To achieve narrow SS molecular weight distributions, fractionated samples of PTMO1546 were prepared. A broad polydispersity was achieved by blending soft segments of different molecular weights (PTMO650, PTMO1000, and PTMO2000). Because of the greater flexibility of the DBDI molecule arising from its −(CH2)2−, regardless of the degree of 2321

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(16) Scortanu, E.; Prisacariu, C.; Caraculacu, A. A.; Bruma, M.; Sulitanu, N. New Heterocyclic Polyurethane-ureas based on 4,4′dibenzyl diisocyanate, Part 1: Influence of Oxadiazole Structure on Mechanical Properties and Cure Conditions. High Perform. Polym. 2006, 18 (2), 127. (17) Prisacariu, C. Polyurethane Elastomers: From Morphology to Mechanical Aspects; Springer: Berlin, Heidelberg, 2011.

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S Supporting Information *

This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +40-32-217454. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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dx.doi.org/10.1021/ie303212s | Ind. Eng. Chem. Res. 2013, 52, 2316−2322