A polarized Infrared Spectroscopy Studies of Waterborne Polyurethane Orientation

97Polyurethanes（PU）are unique polymeric material with wide range of chemical and physical properties. These materials meet highly diversifled demands of modern applications，such as shape memory polymers，adhesives，foams，flber，thermoplastic elastomers and coatings[1][2]. The speciflc structural features of PU istheir two-phase microstructure which arises from the thermodynamic incompatibility between urethane hard segments and polyether / polyester soft segment. Compared with solvent based polyurethane，waterborne polyurethane（WPU）with water as solvent has many advantages，such as clean，safe and reliable，excellent mechanical properties，compatibility，ease of modification and so on [3]. Polycarbonate WPU have many excellent properties，e.g. its hydrolysis resistance，aging resistance，oil resistance and abrasion resistance [4]. But it have poor resilience. This prevents its application in some parts，however，we can change this shortcoming by adding polyether diol.Infrared dichroism is the use of polarized infrared spectromet ry，which can detect the specific orientation of electric dipole mo ment selectively during vibration process. This method can not only measure the type and extent of orientation，but also research on the deformation mechanism of polymers under loads. In this paper，by using the Infrared dichroism，the elastic restitution and orientation of WPU was studied.1 Experimental 1.1 MaterialsPolycarbonate diols（PCDL，CP）；Polyoxytetramethylene （PTMG，CP）；Isophorone diisocyanate（IPDI，CP）；Dimethylol propionic acid（DMPA，AR）；Triethylamine （TEA，AR）；Ethylenediamine（EDA，AR）；N-Methyl pyrrolidone（NMP，AR）；Acetone（AR）.1.2 Method1.2.1 Synthesis of the waterborne polyurethane emulsionWaterborne polyurethane emulsion were prepared by the ratio shown in Table 1. Firstly，PCDL and DMPA dissolved in NMP were added into a four-neck flask equipped with a mechanical stirrer，thermometer，condenser and nitrogen gas inlet，and heated at 60℃ with stirring for 30 min to obtain a homogeneous mixture. IPDI was then added into the homogenized mixture（heating up to 85℃）and stirred under nitrogen atmosphere until the amount of residual NCO groups reached the desired value. Then，acetone was added to adjust the viscosity. Afterwards，the obtained NCO-terminated prepolymer was cooled down to 60℃and neutralized by TEA（DMPA equiv）.Subsequently，the prepolymer solution was emulsifled with a certain amount of deionized water to obtain a prepolymer dispersion. The post-chain extension was carried out with EDA at 60℃ for 30 min. Finally，The WPU dispersions with solid content of about 30 wt% were obtained after the acetone was removed.Table1 Recipe for the preparation of polyurethane dispersionsIPDI（mol）PCDL（mol）PTMG（mol）DMPA（mol）PC-1003101PC-5030.50.511.2.2 Preparation of WPU fllmsThe fllms were obtained by pouring the WPU dispersions onto a Teflon disk to dry at room temperature for 48 h and then at 50℃ in a vacuum dry oven for 48 h to remove the solvent completely. Thickness of the film were prepared 0.1mm（tensile test）and 10µm（FT-IR testing），respectively.1.3 Elastic recovery measurementFor universal tensile testing machine（DSC-300Tensile machine）measurements，the WPU fllms were cut in strips of dimension（50×10）mm 2. Then accurate measurement of the thickness of film by micrometer. Set gauge to 20 mm，and elongation percentage were 100%，150%，200%，250%，300%，r e s p e c t i v e l y. T h e r a t e o f s t r e t c h a n d recovery was 50mm/min. Function of elastic recovery was（1）L 1：total length of sample after elongation，L 2：the length ofthe sample after recovery，L 0：initial length of sample.1.4 Infrared measurementsFT-IR spectra of the WPU thin films were obtained on a Nicolet MAGNA-IR 550 spectrometer. Each sample was scanned 64 times with resolution，4 cm -1 and averaged to obtain the spectrum. All the spectra were scanned within the range 400-4000 cm -1.2 Results and discussion2.1 Influence of polyether glycols to elastic recovery of WPUThe mechanical properties of WPU are closely related to their composition and morphology. The results of mechanical properties testing shown that elastic recovery correlate with the type and content of soft segment. WPU with polyether glycols showed better elastic recovery. In WPU，carbonyl group in soft-segment form hydrogen bonds with the N-H bond more easily，which improve the compatibility between the hard-segment and soft-segment. This makes the microcrystalline area becomes irregularly. ForA polarized Infrared Spectroscopy Studies of Waterborne Polyurethane OrientationJin Guangkui，Cui Liyan，Yao HongweiCollege of Chemical Science and Engineering ，Qingdao University ，Qingdao 266071，P . R. ChinaAbstract：In this article，we prepare waterborne polyurethane（WPU）by polycarbonate diol，polyoxytetramethylene，dimethylol propionic acid and isophorone diisocyanate.We studied the stretch-oriented behavior of waterborne polyurethane films by universal tensile testing machine and FT-IR. And we analyzed the behavior of polyurethane orientation by infrared dichroism spectroscopy.Keywords：waterborne polyurethane；orientation；dichroic ratio98the above reasons，motion of the molecular chain is restricted，and shown poor recovery performance after orientation. Polyether glycol molecular chain have a large amount of ether bond which have low cohesive energy，and ether bond are easier to rotate. For these reasons，motion of the molecular chain is restricted smally. Orientation was disappeared faster after removal of the external force.2.2 FT-IR orientation analysisPolarized infrared is commonly used for the generation of polarized light with an electric field vector oscillating parallel or vertical with respect to the substrate normal，respectively. The experimental dichroic ratio（R）measured by FT-IR can be expressed by the function（2）：（2）where A ∥ and A ⊥ are integrated absorbances of the investigated band measured with the light polarized parallel and perpendicular to the stretching direction，respectively. R value is mainly determined by two parameters，that is the degree of orientation of the molecular chains along the stretch direction and the angle between Vibrational transition direction and the molecular chain axis.Fig.1 Infrared polarization spectrum of the PC-50 WPU fllms（draw ratio was 250%）Fig. 1 shows polarized infrared spectrum of polyurethane film. Obviously，WPU film with orientation has dichroism. The spectra are characterized by bands at 1743 cm -1is attributed to stretching vibration of the carbonyl in polycarbonate（nonbonded）. Its behavior can be on behalf of the polycarbonate soft-segments. Bands at 1720 cm -1 is attributed to stretching vibration of the carbonyl in carbamate（nonbonded），1701 cm -1 is attributed to stretching vibration of the carbonyl which bonding with N-H. Characteristic spectral absorbance at 1257cm -1 is attributed to stretching vibration of the C-N bond in carbamate [5]. Its behavior can be on behalf of the polycarbonate hard-segments.Before stretching，soft segment and hard segment was disordered arrangement in the molecule. In this case，absorbance of the infrared polarization is the same in all directions. Therefore，infrared dichroic ratio R = 1. Molecular chain occur orientation along the drawing direction，when the WPU film stretched in a certain direction. At this time，electric dipole moment of double bond in carbonyl（1743 cm -1）perpendicular to the direction of molecular chain orientation，so that R＜1. As shown in Fig. 2（A，C），with the increase in the degree of orientation，R value becomesgradually smaller. Under the same stretching ratio，the R of PC-50 less than the PC-100. In addition，Electric dipole moment of C-N bond in urethane（1257cm -1）parallel to the direction of molecular chain orientation in the orientation state. so that R＞1.As shown in fig. 2（B，D），infrared dichroic ratio（R）of carbonyl（1743 cm -1）is still less than one，when the external force was removed. We can see from this phenomenon oirientation of the soft segment was still preserved partially in a short time after removal of the external force. This phenomenon has been reflected in the C-N groups of carbamate. Meanwhile，from the Fig.2（B，D），we can also find the R of WPU with polyether glycol is close to 1 than the R of polycarbonate WPU. This phenomenon shows that molecular chain of WPU with polyether glycol is closer to disordered state（R=1）in the relaxed state. That is consistent with the conclusion，which ether bond is good for recovery，in front of the paper.Fig.2 The change of infrared dichroic ratio（R）with draw ratio in stretched state（Fig. A is C=O；Fig. C is C-N）；The change of infrared dichroic ratio（R）with draw ratio in relaxed state（Fig. B is C=O；Fig. D is C-N）3 ConclusionsElastic recovery of polycarbonate WPU was improved effectively with the adding of polyether glycol（PTMG）. The change of dichroic ratio（R）of the film at 1743cm -1and 1257cm -1 proved this point. Infrared dichroic technology has become an effective means of polyurethane-oriented research.References[1]D.K. Chattopadhyay，K.V.S.N. RajuStructural engineering of polyurethane coatings for high performance applications. Prog. Polym. Sci. 32（2007）352-418[2]Ji FL，Hu JL，Li TC，Wong YW. Morphology and shape memory effect of segmented polyurethanes. Part I ：with crystalline reversiblephase. Polymer 2007；48（17）：5133-45.[3]张立武. 水基胶黏剂. [M]. 北京. 化学工业出版社. 2002：159-160.[4]Zhang liwu. water-based adhesives.[M].Beijing ：Chemical Industry Press，2002：159-160.[5]Krajnik JM，Olesen KR，Vandezande GA. Waterborne coating having improved chemical resistance. US patent 6869996，2005.[6]Aswini K. Mishra，D.K. Chattopadhyay，B. Sreedhar，K.V.S.N. Raju. FT-IR and XPS studies of polyurethane-urea-imide coatings. Progress in Organic Coatings 55（2006）231-243.w a ve n u m b e r(cm -1)AD i c h r o i c r a t i o (R )0.90.90.90.91.0BD i c h r o i c r a t i o (R )d r a w r a tioDd r a w r a tio。