Polymers 2015, 7, 1755-1766; doi:10.3390/polym7091481
OPEN ACCESS
polymers ISSN 2073-4360 www.mdpi.com/journal/polymers Article
Study on the Microstructure of Polyester Polyurethane Irradiated in Air and Water Qiang Tian 1,2 , Erzsébet Takács 3 , Ivan Krakovský 4 , Zsolt Endre Horváth 3 , László Rosta 2 and László Almásy 2, * 1
Key Laboratory of Neutron Physics and Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang 621999, China; E-Mail:
[email protected] 2 Wigner Research Centre for Physics, Budapest H-1525, Hungary; E-Mail:
[email protected] 3 Centre for Energy Research, Budapest H-1525, Hungary; E-Mails:
[email protected] (E.T.);
[email protected] (Z.E.H.) 4 Department of Macromolecular Physics, Faculty of Mathematics & Physics, Charles University, Prague 180 00, Czech Republic; E-Mail:
[email protected] * Author to whom correspondence should be addressed; E-Mail:
[email protected]; Tel.: +36-1392-2222 (ext. 1447); Fax: +36-1392-2548. Academic Editor: Yue Zhao Received: 13 August 2015 / Accepted: 2 September 2015 / Published: 15 September 2015
Abstract: The gamma irradiation induced aging of thermoplastic polymer Estane 5703 in air and water environments was studied by small-angle neutron scattering (SANS), Fourier transform infrared spectroscopy (FTIR), gel permeation chromatography (GPC), and X-ray diffraction (XRD). The degree of phase mixing was increased after irradiation, accompanied by the increase of domain distance and decrease of domain size. The hard domain distance increased from 9.8 to 11.2 nm and 14.4 nm for the samples irradiated in air and water with a dose up to 500 kGy, respectively. The GPC results indicated progressive formation of larger linked structures with very high molar mass with increasing absorbed doses. The samples irradiated in water exhibited a stronger aging effect than those irradiated in air. The FTIR results suggested that the cross-linking occurred among the secondary alkyl radicals, and the interactions in hard domains weakened because of the loss of inter-urethane H-bonds. The volume fraction of well-ordered soft segments in Estane increased upon irradiation. Keywords: polyurethane; irradiation; microstructure; phase mixing; SANS
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1. Introduction Segmented polyurethanes (PUs) are a class of thermoplastic elastomers that bridge the gap between rubbers and plastics [1]. The superior properties of PUs originate in the thermodynamic incompatibility between the hard segments (HSs) and soft segments (SSs). PUs have wide applications in industrial, aerospace, and biomedical areas. When PUs are exposed to high energy irradiation, the structure and mechanical property may be altered. It is of importance from both basic and applied viewpoints to determine how the microstructure and stability could be influenced by irradiation. Generally, PUs have shown higher radiation resistance compared with other common polymers, such as polyolefins and vinyl polymers [2,3]. However, the effects of irradiation on the microstructure are complex, and closely related to the chemical composition, irradiation environment, and absorbed dose. Shintani et al. [4] found that PUs chain extended with 1,4-butanediol (BDO) degrade, while those without BDO mainly undergo cross-linking by γ-ray irradiation. Pierpoint et al. [5] reported that Estane 5703 primarily undergoes rapid cross-linking, and partially undergoes scission reactions both in the presence and absence of oxygen after γ or electron irradiation. Murray et al. [6] revealed a high level of cross-linking at 200 kGy and an increase in phase segregation of Pelletane 2363-90A after irradiation. Burillo et al. [7] found that the tensile strength of polyester polyurethane was reduced by 50% with 310 kGy absorbed dose, and adding diphenylbutadiyne could effectively suppress the polymer chain degradation. Sui et al. [8] studied γ-irradiation effects on a PU adhesive and revealed the degradation in the SSs composed of poly(ethyleneglycol adipate) with the dose up to 1000 kGy. As it is characteristic for polymers, chain scission and cross-linking are the two main reactions in PUs exposed to γ irradiation. It is widely accepted that the unique properties of PUs are closely related to their multi-phase microstructure, in which the hard domains act as physical cross-linking points and reinforcing fillers embedded in the soft matrix [9–11]. Therefore, the performance of irradiated PUs is not only determined by the chemical composition, but also strongly influenced by the microphase structure, such as domain size, spatial distribution, and degree of phase separation. From the point of view of applications of PUs it is vital to understand these effects. The small-angle scattering technique is ideally suited for investigating nanoscale morphology of materials. Morphological changes due to chemical reaction, microphase separation kinetics, temperature, and mechanical deformation can be effectively observed by scattering methods [12–16]. Up to now, little information is published on the microphase structure of irradiated PUs. In this work, we focus on studying the domain structure of Estane (a commercially available thermoplastic PU) γ-irradiated both in air and water using small-angle neutron scattering (SANS) technique. Additionally, information on cross-linking, chemical reactions, and crystallization of SS is obtained by gel permeation chromatography (GPC), Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD). A phase mixing behavior, for the first time, is observed in this class of polyester polyurethane systems. The effects of the water and air environments during irradiation on the microstructure of Estane are comparatively discussed.
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2. Experimental Section 2.1. Materials 2.1. Materials Estane Inc., Westerlo-Ovel, Westerlo-Ovel, Estane 5703 5703 was was obtained obtained as as pellets pellets from from the the Lubrizol Lubrizol Advanced Advanced Materials, Materials, Inc., Belgium. It is 2323 wt wt % HSs, prepared by the Belgium. It is aa segmented segmentedpolyurethane polyurethanecopolymer, copolymer,containing containingabout about % HSs, prepared by 1 reaction of 4,4 -diphenylmethane diisocyanate (MDI) andand poly(butylene adipate) (PBA) with BDO as the reaction of 4,4′-diphenylmethane diisocyanate (MDI) poly(butylene adipate) (PBA) with BDO the chain extender. TheThe SSs SSs composed of PBA have have an average molecular weightweight in the range 800 to as the chain extender. composed of PBA an average molecular in the of range of 1050 Da. The molecular structure of Estane is presented in Figure 1. 800 to 1050 Da. The molecular structure of Estane is presented in Figure 1.
Figure 1. Molecular structure of Estane 5703. Figure 1. Molecular structure of Estane 5703. Estane pellets were compression-molded into 1 mm thick film at 20 MPa, 120 ˝°C C for 10 min and then cooled to room room temperature temperature(RT), (RT),initiating initiatingthe thephase phaseseparation separationprocess. process. The samples were stored at The samples were stored at RT RT a drying oven two years beforeirradiation. irradiation.The TheEstane Estanesamples sampleswere wereirradiated irradiated both both in in air and in aindrying oven forfor two years before 60 distilled water 500 kGy with a dose raterate of water using using 60Co Coγγ radiation radiationsource sourceatatdoses dosesofof50, 50,100, 100,200, 200,300 300and and 500 kGy with a dose 3.9 kGy/h at Centre forfor Energy Research, Budapest, Hungary. of 3.9 kGy/h at Centre Energy Research, Budapest, Hungary.The Thesamples samples(10 (10mm mmˆ×10 10mm mmˆ× 11 mm) were placed in glass tube holders with diameter of 20 mm and wall thickness of about 1 mm, filled with air or distilled water. water. 2.2. Characterization 2.2. Characterization The The SANS SANS measurements measurements were were performed performed on on the the small-angle small-angle neutron neutron scattering scattering diffractometer diffractometer Yellow Submarine at the Budapest Research Reactor (BNC, Budapest, Hungary). The Yellow Submarine at the Budapest Research Reactor (BNC, Budapest, Hungary). The scattering scattering intensity I(q) is is measured measured as as aa function functionof ofscattering scatteringvector vectorqq==4πsinθ/λ, 4πsinθ/λ,where where λ the is the wavelength intensity I(q) λ is wavelength of of incident neutrons, andθ θis ishalf halfofofthe thescattering scatteringangle. angle. The Themonochromatic monochromatic beam beam with with aa mean thethe incident neutrons, and mean wavelength of 0.47 nm and 0.2 FWHM had been produced by a multidisk mechanical velocity selector wavelength of 0.47 nm and 0.2 FWHM had been produced by a multidisk mechanical velocity selector (KFKI, A BF BF3 gas gas filled filled multiwire 64 cm cm sensitive sensitive area multiwire detector detector of of 64 64 cm cm ˆ × 64 area was was (KFKI, Budapest, Budapest, Hungary). Hungary). A ´1−1 placed placed at at distance distance of of 5.5 5.5 mm from fromthe thesample, sample,totocover coverthe theq-interval q-intervalofof0.1–0.9 0.1–0.9nm nm . . The The total total data data acquisition time for each each sample sample was was about about 66 h.h. The The scattering scattering data data were were processed processed using using BerSANS BerSANS software (Hahn-Meitner-Institut, Berlin, Germany) [17]. The data reduction corrects the raw measured Berlin, Germany) [17]. The data reduction corrects the data for the contributions contributions of of the the background, background, transmission, transmission, and and scattering scattering from from the the tape tape used used for fixing the samples. All the data were fitted by SASfit software (Paul Scherrer Institute, Villigen, Switzerland), and the q-resolution has been taken into account by convoluting the theoretical curve with the instrument resolution function [18]. The gel permeation chromatography (GPC) measurements were performed using a Biospher GMB 100 column column(8(8 ˆ mm, 500Labio, mm, Czech Labio,Republic) Czech Republic) filled with 10particles. µm sorbent particles. mmmm × 500 filled with 10 μm sorbent Tetrahydrofuran Tetrahydrofuran Louis, MO, USA), distilled dried over molecular (4 Å), (Sigma-Aldrich, (Sigma-Aldrich, St. Louis, MO, St. USA), distilled and dried overand molecular sieves (4 Å), sieves was used as
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used phase as a mobile phaserate at a1 flow rate 1Small mL/min. Small toluene (2% (w/v)) used as awas mobile at a flow mL/min. amount ofamount tolueneof(2% (w/v)) was usedwas as internal internal standard. Concentration all PU solutions to GPCwas analysis 2.00% The standard. Concentration of all PUof solutions subjectedsubjected to GPC analysis 2.00%was (w/v). The(w/v). data from from a index refractive index detector were treated using CSW( DataApex, 1.7 software (DataApex, adata refractive detector were collected andcollected treated byand using CSWby1.7 software Prague, Czech Prague, Czech Republic). For the determination of molar masses, a universal calibration equation Republic). For the determination of molar masses, a universal calibration equation calculated from the calculated from thestandards data on polystyrene standardsService) (Polymer wasforused. The lower data on polystyrene (Polymer Standards wasStandards used. TheService) lower limit resolution (total limit for resolution (total exclusion limit) of theused chromatographic column used isthat about 9.5 mL. This exclusion limit) of the chromatographic column is about 9.5 mL. This means polymer species means or thatbranched) polymer with species (linear branched) with molar mass higher than ca. 1,000,000 g/mol (linear molar massorhigher than ca. 1,000,000 g/mol are not resolved by the column. are not resolved the column. Fourier (FTIR) transform spectroscopy (FTIR) Research was takenSeries by a Unicam Fourier transformbyinfrared spectroscopy wasinfrared taken by a Unicam Mattson 1 FTIR Mattson Research Series 1 FTIR London, spectrometer Mattson, London,ofUK). The crystalline structure spectrometer (Unicam Mattson, UK).(Unicam The crystalline structure the samples was determined of the samples determined byDiscover XRD, using a Bruker AXS D8 Discover diffractometer (Bruker AXS, by XRD, using was a Bruker AXS D8 diffractometer (Bruker AXS, Karlsruhe, Germany) equipped Karlsruhe, Germany) equipped with a Göbel mirror with a Göbel mirror and a scintillation detector usingand CuaKscintillation α radiation. detector using Cu K α radiation. Results 3. Results Small-Angle Neutron Neutron Scattering (SANS) 3.1. Small-Angle The I(q) curves and irradiated Estane are are shown in Figure 2. All curves of of original original(labeled (labeledasas0 0kGy) kGy) and irradiated Estane shown in Figure 2. the curves display a broad interference peak, which suggests a broader distribution of the interdomain All the curves display a broad interference peak, which suggests a broader distribution of The peaks peaks shift shift toward toward lower lower q and become less distinct with increasing distance throughout the samples. The the absorbed dose, indicating that the interdomain association is weakened and the most probable domain distance is increased.
Figure 2. SANS data of the γ-ray irradiated Estane samples at absorbed doses of 0, 100, Figure 2. SANS data of the γ-ray irradiated Estane samples at absorbed doses of 0, 100, 200 200 and 500 kGy in air (a) and water (b). and 500 kGy in air (a) and water (b). In order order to to get get more more information, information, we we applied applied the the Debye-Anderson-Brumberger Debye-Anderson-Brumberger (DAB) (DAB) form form factor factor In P(q, aacor plus Percus-Yevick Percus-Yevick (PY) (PY) structure structure factor factor as as an an approximate approximate model model to to fit fit the the scattering scattering cor)) plus P(q, intensities by by equation: equation: intensities I pqq “ K ˆ P pq, acor q ˆ S pq, RHS , vq ` IB (1) (1) q , , , where RHS and v denote the hard sphere interaction radius and volume fraction, acor is the length over where RHS and v denote the hard sphere interaction radius and volume fraction, acor is the length over which the structural correlation decays and can be considered as the average size of hard domains which the structural correlation decays and can be considered as the average size of hard domains because of the low HSs content in Estane, K is a scale factor, and I B is a constant which accounts because of the low HSs content in Estane, K is a scale factor, and IB is a constant which accounts for for the incoherent scattering [16,19–21]. The fitting parameters are listed in Table 1. The hard domain the incoherent scattering [16,19–21]. The fitting parameters are listed in Table 1. The hard domain
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Polymers 2015, 7 5 distance, expressed by 2RHS , increases from 9.8 to 11.2 nm and 14.4 nm for the samples irradiated in air and water with a dose up to 500 kGy (Figure 3), respectively. Hence the samples irradiated in water distance, expressed by 2RHS, increases from 9.8 to 11.2 nm and 14.4 nm for the samples irradiated in air exhibit stronger structure changes. The number density of the hard domains, proportional to v/RHS 3 , and water with a dose up to 500 kGy (Figure 3), respectively. Hence the samples irradiated in water is decreased gradually with dose (Table 1). The results suggest that the increase in domain distance exhibit stronger structure changes. The number density of the hard domains, proportional to v/RHS3, is is caused by the decrease in number density of the hard domains. The hard sphere volume fraction v decreased gradually with dose (Table 1). The results suggest that the increase in domain distance is decreases with increasing dose as well, indicating that a part of HSs are dissolved into soft domains caused by the decrease in number density of the hard domains. The hard sphere volume fraction v induced by irradiation. decreases with increasing dose as well, indicating that a part of HSs are dissolved into soft domains induced by irradiation. Table 1. Structural parameters obtained from SANS data on irradiated Estane samples by curve-fitting using the DAB-PY model. Table 1. Structural parameters obtained from SANS data on irradiated Estane samples by curve-fitting Samplesusing the DAB-PY RHS (nm)model. acor (nm) v v/RHS 3 (nm´3 ) Samples Original Original 100 kGy in air 100 200 kGy kGy in in air air 200 kGy in air 500 kGy in air 500 kGy in air 100 kGy in water 100 kGy in water 200 kGy in water 200 kGy in water 500 kGy kGy in in water water 500
R (nm) HS˘ 4.9 0.1 4.9 ± 0.1 5.0 ˘ 0.1 5.0 ˘ ± 0.1 5.5 0.2 5.5 ± 0.2 5.6 ˘ 0.5 5.6 ± 0.5 5.4 ˘ 0.1 5.4 ± 0.1 6.2 ˘ 0.1 6.2 ± 0.1 7.2 0.2 7.2 ˘ ± 0.2
acor ˘ (nm) 1.8 0.3 1.8 ± 0.3 1.7 ˘ 0.2 1.7 ˘ ± 0.2 1.0 0.2 1.0 ± 0.2 0.9 ˘ 0.3 0.9 ± 0.3 1.1 ˘ 0.1 1.1 ± 0.1 0.8 ˘ 0.1 0.8 ± 0.1 1.0 0.1 1.0 ˘ ± 0.1
v 0.03 0.21 ˘ 0.21 ˘ ± 0.03 0.18 0.02 0.18 ± 0.02 0.10 ˘ 0.02 0.10 ± 0.02 0.05 ˘ 0.02 0.05 ± 0.02 0.10 ˘ 0.02 0.10 ± 0.02 0.08 ˘ 0.01 0.08 ± 0.01 0.09 0.01 0.09 ˘ ± 0.01
3 −3 v/R 1.8HSˆ (nm 10´3 ) −3 1.8 ×10 10´3 1.4ˆ −3 1.4× 10 10´4 6.0ˆ 6.0× 10−4 2.8 ˆ 10´4 2.8 × 10−4 6.4 ˆ 10´4 6.4 × 10−4 3.4 ˆ 10´4 3.4 × 10−4 ´4 2.4 2.4ˆ× 10 10−4
Figure 3. Variation of the hard domain distance (2RHS ) with the absorbed dose, as obtained Figure 3. Variation of the hard domain distance (2RHS) with the absorbed dose, as obtained by SANS model fitting. by SANS model fitting. 3.2. Gel Permeation Chromatography Chromatography (GPC) (GPC) On the at at 15 15 mLmL of elution volume is from the the GPC GPC chromatograms chromatograms shown shownininFigure Figure4,4,thethepeak peak of elution volume is from internal standard (toluene). TheThe peak at at 12.2 mL the internal standard (toluene). peak 12.2 mLfound foundfor forthe theoriginal originalEstane Estanecorresponds correspondsto to aa molar mass of 20,000 upup to 100 kGy, the the position of the 20,000 g/mol. g/mol. For For the thesamples samplesirradiated irradiatedininairairwith witha dose a dose to 100 kGy, position of peak remains constant, however, the peak intensity decreases. Simultaneously, a shoulder on the the peak remains constant, however, the peak intensity decreases. Simultaneously, a shoulder onleft theside left of (at lower elution volume) is noticed. It can bebeconcluded sidethe ofpeak the peak (at lower elution volume) is noticed. It can concludedthat thata adose doseofofless lessthan than 100 100 kGy causes a conversion of a relatively relatively small small amount amount of of original original chains chains into into larger larger linked linked structures. structures. For a dose higher than 200 kGy, there is a slight shift of the peak to the right and also a significant increase of the signal at the left side of the peak corresponding to the formation of a large amount of linked structures
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dose higher2015, than 7200 kGy, there is a slight shift of the peak to the right and also a significant increase of6 Polymers the signal at the left side of the peak corresponding to the formation of a large amount of linked structures with with very very high high molar molar mass. mass. Samples Samples irradiated irradiated with with aa dose dose of of 300 300 kGy kGy are are still still soluble soluble in in tetrahydrofuran tetrahydrofuran (THF) however the samples irradiated with a dose up to 500 kGy are insoluble in THF (it (THF) however the samples irradiated with a dose up to 500 kGy are insoluble in THF (it swells swells in in THF, THF, only). This occurs occurs because only). This because cross-linking cross-linking of of PU PU chains chains exceeds exceeds aa critical critical point point when when three-dimensional three-dimensional network network structure structure of of macroscopic macroscopic size size appears appears in in the the system. system. Therefore, Therefore, the the critical critical value value of of the the dose dose for for polymer polymer network network formation formation is is somewhere somewhere between between 300 300 and and 500 500 kGy. kGy. For For the the samples irradiated irradiated in water, water, the situation is very similar to the samples irradiated in air. The only difference is that the shoulders the situation is very similar to the samples irradiated in air. The only difference is that the shoulders at at lower lower elution elution volume volume shift shift more more to to the the left, left,indicative indicativeof ofthe theformation formationof oflonger longerchains. chains.
Figure 4. GPC curves of the Estane samples irradiated in air (a) and water (b). Figure 4. GPC curves of the Estane samples irradiated in air (a) and water (b). 3.3. 3.3. Fourier Fourier Transform Transform Infrared Infrared Spectroscopy Spectroscopy(FTIR) (FTIR) The thethe irradiated Estane is further studied by FTIR by for its sensitivity the chemical The microstructure microstructureof of irradiated Estane is further studied FTIR for itstosensitivity to changes, hydrogen bonding, and molecular chain packing in PUs. The FTIR spectra of the samples the chemical changes, hydrogen bonding, and molecular chain packing in PUs. The FTIR spectra of ´l −l irradiated to 500 kGy are in the 1000 to 1800 intoFigure the samples irradiated to shown 500 kGy are region shownfrom in the region fromcm 1000 1800 5.cmThe inabsorption Figure 5. ´1 −1 peak at 1414 cmpeak corresponds stable C–C of the aromatic and all the corresponds to stretching the stable vibration C–C stretching vibrationring, of the aromatic The absorption at 1414 cmto the ´1 −1 data thisnormalized peak [22].toThe vibration band atvibration 1255 cmband shows a loss ring,are andnormalized all the datatoare thisCH peak [22]. The CH2 wagging at 1255 cmin 2 wagging intensity afterinirradiation to a irradiation dose of 500tokGy. means that the number CHnumber decreased shows a loss intensity after a doseIt of 500 kGy. It means thatofthe ofisCH 2 groups 2 groups because the γbecause irradiation generatecould the secondary alkyl radicals and cross-linking among is decreased the could γ irradiation generate the secondary alkylthe radicals and theoccurs cross-linking ´1 −1 the secondary radicals [5]. IR bands centered at 1529 and 1216 cmandare assigned toassigned the free occurs among alkyl the secondary alkylThe radicals [5]. The IR bands centered at 1529 1216 cm are v(C–N) + δ(N–H) [23]. inTherefore, increase of two bands for two the irradiated can be to the free v(C–N)in+HSs δ(N–H) HSs [23].the Therefore, thethe increase of the bands for Estane the irradiated attributed theattributed loss of inter-urethane That is, upon irradiation, the ordered molecular chain Estane cantobe to the loss ofH-bonds. inter-urethane H-bonds. That is, upon irradiation, the ordered structures by H-bonds in HSs destroyed. molecularinduced chain structures induced by are H-bonds in HSs are destroyed. 3.4. 3.4. X-Ray X-Ray Diffraction Diffraction (XRD) (XRD)
The XRD XRD patterns patterns of of the the samples samples irradiated irradiated in in air air and and water water are are shown shown in in Figure Figure6.6. An An amorphous amorphous The ˝ ˝ ˝ ˝ halo and and four four diffraction diffraction peaks peaks located located at at 9.4 9.4°,, 21.6 21.6°,, 22.4 22.4° and and 28.6 28.6° are are observed observed for for all all the the samples. samples. halo The only was thethe increase of the intensity of theofpeak The only significant significant change changeobserved observedininthe theseries series was increase of relative the relative intensity the at 21.6°, corresponding to a Bragg spacing of 4.11 Å, which relates to a certain degree of structural ordering upon irradiation. The α-form monoclinic cell of pure PBA exhibits two characteristic (110) and
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peak at 21.6˝ , corresponding to a Bragg spacing of 4.11 Å, which relates to a certain degree of structural 2015, ordering Polymers 7 upon irradiation. The α-form monoclinic cell of pure PBA exhibits two characteristic 7 ˝ ˝ Polymers 7 (110) and 2015, (020) reflections centered at 21.75 and 21.4 [24,25], and the similar reflections from the SSs7 consisting of poly(ethylene or poly(tetramethylene reportedfrom in [26,27]. (020) reflections centered at adipate) 21.75° and 21.4° [24,25], and theadipate) similar are reflections the SSs Therefore, consisting (020) reflections centered at 21.75° and 21.4° [24,25], and the similar reflections from the SSs consisting thepoly(ethylene results implyadipate) that theor volume fraction of well-ordered SSs in Estane is increased by of poly(tetramethylene adipate) are reported in [26,27]. Therefore,induced the results of poly(ethylene adipate) or poly(tetramethylene adipate) are reported in [26,27]. Therefore, the results cross-linking. imply that the volume fraction of well-ordered SSs in Estane is increased induced by cross-linking. imply that the volume fraction of well-ordered SSs in Estane is increased induced by cross-linking.
Figure 5. FTIR spectra of the Estane samples irradiated at the dose of 500 kGy in air Figure 5. FTIR spectra of the Estane samples irradiated at the dose of 500 kGy in air and water. and water. Figure 5. FTIR spectra of the Estane samples irradiated at the dose of 500 kGy in air and water.
Figure and 500 500 kGy kGy Figure 6. 6. XRD XRD patterns patterns of of the the Estane Estane samples samples irradiated irradiated at at doses doses of of 0, 0, 100, 100, 200 200 and Figure 6. XRD patterns of the Estane samples irradiated at doses of 0, 100, 200 and 500 kGy in in air air and and water. water. in air and water. 4. Discussion 4. Discussion Discussion 4. 4.1. Phase Mixing Induced by γ Irradiation 4.1. Phase Phase Mixing Mixing Induced Induced by by γγIrradiation Irradiation 4.1. The hard and soft domains in PUs are usually thermodynamically incompatible at ambient The hard hard and and soft soft domains in in PUs PUs are are usually usually thermodynamically incompatible incompatible at at ambient ambient The temperatures. The phasedomains separation process is inhibitedthermodynamically by steric and molecular mobility constraints. temperatures. The The phase phase separation separation process process is is inhibited inhibited by by steric steric and and molecular molecular mobility mobility constraints. temperatures. constraints. However, in the current work, the SANS results indicate that the γ irradiation leads to an apparent However, in the current work, the SANS results indicate that the γ irradiation leads to an apparent However, in the current work, the SANS results indicate that the γ irradiation leads to an apparent increase of compatibility between the HSs and the SSs. It can be attributed to two factors. increase of compatibility between HSs SSs. It attributed to factors. increase of hand, compatibility between the HSs and and thethat SSs. It can can be be attributedboth to two two factors. On one the GPC results the clearly showthe samples irradiated in air and water mainly On one hand, the GPC clearly that (R–CH samples2–CH irradiated both2–R) in air and water mainly undergo cross-linking. The results secondary alkylshow radicals induced by radiation 2–ĊH–CH undergo cross-linking. secondary (R–CH inducedthebymolecules radiation 2–CH 2–ĊH–CH 2–R) undergo cross-linking, The leading to new alkyl bondsradicals between Estane molecules and bringing
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On one hand, the GPC results clearly show that samples irradiated both in air and water mainly ˙ undergo cross-linking. The secondary alkyl radicals (R–CH2 –CH2 –CH–CH 2 –R) induced by radiation undergo leading to new bonds between Estane molecules and bringing the molecules8 Polymerscross-linking, 2015, 7 closer to each other. This process is shown as follows:
The above above reaction reaction can can take take place place both both in in SSs SSs (PBA) (PBA) and and HSs HSs (BDO). (BDO). The The HSs HSs volume volume fraction fraction is is The much lower lower than than SSs SSs in in Estane Estane (Figure (Figure 1). 1). Hence, Hence, the the cross-linking cross-linking occurred occurred in in SSs SSs is is the the major major reaction, reaction, much resulting in in the the increase increase of of the the contractile contractile force force in in SSs. SSs. resulting On the the other other hand, hand, the the γγ radiation radiation may may lead lead to to the the degradation degradation of of the the HSs. HSs. It reported that that On It was was reported 4,4′-methylenedianiline (MDA) is is produced produced in in MDI-based MDI-based PUs PUs subjected subjected to to γγ irradiation irradiation with with doses doses up up 1 4,4 -methylenedianiline (MDA) to 100 100 kGy kGy [28]. [28]. It the urethane urethane linkage linkage proximal proximal to to terminal terminal amino amino to It means means that that the the irradiation irradiation cleaves cleaves the groups. This of chain chain mobility mobility of of HSs, HSs, and and weaken weaken groups. This chain chain scission scission reaction reaction could could result result in in the the increase increase of the molecular molecular interactions interactions among among hard hard domains. domains. Therefore, Therefore, it it appears appears that that an an increase increase in in the the contractile contractile the force between between the the SSs SSs induced induced by by cross-linking cross-linking tries tries to to pull pull aa part part of of HSs HSs into into the the soft soft domain domain matrix matrix force and break break down down the the hard hard domains. domains. Smaller Smaller hard hard domains domains may may be be totally totally dissolved dissolved into into the the soft soft domains domains and and then the average size of the hard domains would decrease. The thermodynamic driving force for and then the average size of the hard domains would decrease. The thermodynamic driving force for the the phase separation is gradually reduced with increasing dose. Hence, the degree of phase mixing in phase separation is gradually reduced with increasing dose. Hence, the degree of phase mixing in the the irradiated Estane is enhanced, accompanied by the decrease of acor and increase of RHS. irradiated Estane is enhanced, accompanied by the decrease of acor and increase of RHS . Morphological changes, such as phase separation or mixing of PUs induced by irradiation, are rarely Morphological changes, such as phase separation or mixing of PUs induced by irradiation, are rarely reported. Murray et al. [6] and Huang et al. [29] studied the phase separation behavior of polyether PUs reported. Murray et al. [6] and Huang et al. [29] studied the phase separation behavior of polyether irradiated by electron beam. By using XRD and DSC techniques, their results showed an increase in PUs irradiated by electron beam. By using XRD and DSC techniques, their results showed an increase phase separation during the exposure of irradiation. Walo et al. [30] studied the effects of HS/SSs in phase separation during the exposure of irradiation. Walo et al. [30] studied the effects of HS/SSs composition on the structure and mechanical properties of polyester PUs exposed to electron beam composition on the structure and mechanical properties of polyester PUs exposed to electron beam irradiation. They assumed that the experimentally observed deterioration of the mechanical strength irradiation. They assumed that the experimentally observed deterioration of the mechanical strength upon irradiation was related to the reduction of the degree of phase separation. In this work, we identified upon irradiation was related to the reduction of the degree of phase separation. In this work, we identified an increase of phase mixing in Estane exposed to γ-irradiation by analysis of SANS data of the domain an increase of phase mixing in Estane exposed to γ-irradiation by analysis of SANS data of the domain sizes and distribution. Thus, the aging of PUs induced by irradiation is a complex process. Besides sizes and distribution. Thus, the aging of PUs induced by irradiation is a complex process. Besides the the chemical reactions (chain scission and cross-linking), the physical (morphological) changes are chemical reactions (chain scission and cross-linking), the physical (morphological) changes are another another important aspect of the aging process. Comparison with the above mentioned studies shows important aspect of the aging process. Comparison with the above mentioned studies shows that the that the microstructure changes depend on the chemical composition and the type of PUs (polyester or microstructure changes depend on the chemical composition and the type of PUs (polyester or polyether). polyether). Furthermore, the increase in the degree of phase mixing and the decrease in the number Furthermore, the increase in the degree of phase mixing and the decrease in the number density of hard density of hard domains in Estane induced by γ-irradiation would result in the deterioration of domains in Estane induced by γ-irradiation would result in the deterioration of mechanical properties mechanical properties due to the reduction of the number density of reinforce fillers (hard domains), due to the reduction of the number density of reinforce fillers (hard domains), as was shown in a recent as was shown in a recent study [31]. study [31]. 4.2. The of the the Water Water and and Air Air Environments Environments 4.2. The Effects Effects of Aqueous environment effects on on the the microstructure of irradiated polymers. Highly reactive Aqueous environmenthas hasspecific specific effects microstructure of irradiated polymers. Highly ˙ hydroxylhydroxyl radicals (ȮH) can(OH) be generated in the radiolysis of water [32]. The hydroxyl are radicals capable reactive radicals can be generated in the radiolysis of water [32]. Theradicals hydroxyl of abstracting efficiently Estane from polymer chains. It follows are capable of hydrogen abstractingatoms hydrogen atomsfrom efficiently Estane polymer chains.thatIt cross-linking follows that should be highly favoured in aqueous media, i.e., media, i.e., cross-linking should be highly favoured in aqueous
2 Estane + 2 OH
2 HOH + 2 Estane
Estane
Estane
Aqueous environment has specific effects on the microstructure of irradiated polymers. Highly reactive hydroxyl radicals (ȮH) can be generated in the radiolysis of water [32]. The hydroxyl radicals are capable of abstracting atoms efficiently from Estane polymer chains. It follows that cross-linking Polymers 2015,hydrogen 7 1763 should be highly favoured in aqueous media, i.e., Polymers 2015, 7 2 Estane + 2 OH
2 HOH + 2 Estane
Estane
9
Estane
The the cross-linking cross-linking of Estane irradiated higher than than that that irradiated irradiated in air. The degree degree of of the of Estane irradiated in in water water is is higher in air. Accordingly, Accordingly, the the samples samples irradiated irradiated in in water water exhibit exhibit more more significant significant aging aging effects effects on on the the domain domain structures structures (Figure (Figure 3). 3). ´1 The is increased increased for for the the sample sample irradiated irradiated in in air, air, and and remains remains nearly nearly The carbonyl carbonyl C=O C=O peak peak at at 1725 1725 cm cm−1 is unchanged for the sample irradiated in water (Figure 5). Most possibly in the presence of O , the alkyl unchanged for the sample irradiated in water (Figure 5). Most possibly in the presence of O22, the alkyl radicals could react with the O2 to form peroxyl radicals, and subsequently transform to new carbonyls. Although the cross-linking is the main effect induced by irradiation, the alkyl radicals could also react with the O2 . One possible reaction can be expressed as follows [33,34]: R
CH 2
CH 2
CH
CH 2
R
+O 2
O2 R
CH 2
CH 2
CH
CH 2
R
R
HC
O + O
CH 2
R
It has O2Ois2 It has been been reported reported that that the the increase increaseof ofmolecular molecularweight weightofofEstane Estaneirradiated irradiatedininthe theabsence absenceofof O22 during during much larger is much largerthan thanthat thatirradiated irradiatedininthe the presence presence OO22 [5]. [5]. Consequently, Consequently, the the presence presence of of O irradiation results results in in chain chain scission scission reactions reactions in in Estane, Estane, counteracting counteracting the the molecular molecular weight weight increase increase irradiation caused by by cross-linking. cross-linking. caused 5. Conclusions 5. Conclusions
The microstructure microstructure of of Estane Estane γ-irradiated γ-irradiated in studied by by SANS, SANS, GPC, GPC, FTIR, FTIR, The in air air and and water water was was studied and XRD. XRD. The The scattering scatteringdata dataindicated indicatedthat thatthethedegree degree phase mixing in Estane enhanced, and of of phase mixing in Estane waswas enhanced, the the domain decreased, their separation increased afterirradiation. irradiation.The TheGPC GPCresults results demonstrated demonstrated domain sizesize decreased, andand their separation increased after progressive formation crosslinked structures with with very high increasing absorbed progressive formationofoflarger larger crosslinked structures verymolar high mass molarwith mass with increasing dose. When the When accumulated dose exceeded kGy, the insoluble absorbed dose. the accumulated dose 300 exceeded 300materials kGy, the formed materials formed polymer insolublenetworks. polymer Compared with the samples irradiated in air, the samples irradiated in water exhibited stronger aging networks. Compared with the samples irradiated in air, the samples irradiated in water exhibited stronger effects,effects, as seen by anbyincrease of phase mixing. The FTIR cross-linking aging as seen an increase of phase mixing. The FTIRresults resultssuggested suggestedthat that the the cross-linking could react react with with alkyl alkyl radicals radicals to to form form carbonyl carbonyl occurred among among the the secondary secondary alkyl alkyl radicals, radicals, and and the the O O22 could occurred bonds. The results showed the volume fraction of well-ordered soft segments in Estaneinwas increased bonds. TheXRD XRD results showed the volume fraction of well-ordered soft segments Estane was upon irradiation. increased upon irradiation. Acknowledgments Acknowledgments
This work work was was supported supported by by project project KMR12-1-2012-0226 KMR12-1-2012-0226 granted granted by by the the National National Development Development This Agency of of Hungary Hungarythe theNational NationalNatural NaturalScience Science Foundation of China under 11205137 Agency Foundation of China under grantgrant No. No. 11205137 and and 11405152. We thank Éva Koczog for sample preparation andmeasurements. IR measurements. 11405152. We thank Éva Koczog for sample preparation and IR Author Contributions Contributions All listed work. László Almásy andand László Rosta conceived and listed authors authorscontributed contributedtotothetheresearch research work. László Almásy László Rosta conceived directed the research project; LászlóLászló Almásy performed the SANS experiments; Qiang Tian László and directed the research project; Almásy performed the SANS experiments; Qiangand Tian and Almásy proposed ideas for datafor explanation and wrote paper; Erzsébet TakácsTakács conducted the γ László Almásy proposed ideas data explanation and the wrote the paper; Erzsébet conducted irradiation and FTIR Ivan Krakovský did the GPC and analysis;and Zsolt Endre the γ irradiation andanalysis; FTIR analysis; Ivan Krakovský did measurements the GPC measurements analysis; Horváth did Horváth the XRDdid measurements. Zsolt Endre the XRD measurements.
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