Nanostructure Controlled Shape Memory Effect in Polyurethanes

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C: Physical Processes in Nanomaterials and Nanostructures

Nanostructure Controlled Shape Memory Effect in Polyurethanes Arpan Biswas, Vinod Kumar Aswal, Biswajit Ray, and Pralay Maiti J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02824 • Publication Date (Web): 27 Apr 2018 Downloaded from http://pubs.acs.org on April 27, 2018

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The Journal of Physical Chemistry

Nanostructure Controlled Shape Memory Effect in Polyurethanes

Arpan Biswas,1 Vinod K. Aswal,2 Biswajit Ray3 and Pralay Maiti1*

1

School of Materials Science and Technology, Indian Institute of Technology (Banaras Hindu

University),Varanasi 221 005, India 2

Solid State Physics Department, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085,

India 3

Department of Chemistry, Institute of Science, Banaras Hindu University, Varanasi 221 005,

India

*Correspondences should be made to Pralay Maiti ([email protected])

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ABSTRACT: Different hard segment content polyurethanes have been synthesized using linear (hexamethylene

diisocyanate;

HMDI)

and

alicyclic

(isophorone

diisocyanate;

IPDI)

diisocyanates to understand the effect on structure-property relationship and how it affect shape memory behavior of polyurethanes. Structural details have been elucidated through NMR and other spectroscopic techniques including the quantification of hard segment content. The nature of interaction between the polymer chains and its extent has been revealed. Thermal and mechanical properties as a function of chemical structure and hard segment content indicating faster degradation in higher hard segment content. Layer by layer self-assembly through extensive hydrogen bonding has been established through XRD, small angle neutron scattering, AFM and optical images by capturing nanometer scale to micron scale inhomogeneities. The greater interaction in HMDI system as compared to IPDI PUs leads to the crystallization of hard segment. Differential shape memory effect has been reported with varying degree of shape fixity and recovery as a function of hard segment content. The greater retracting force in HMDI system with increasing HSC helps to recover greater percentage of the permanent shape, in contrary, a decreasing shape recovery value is obtained in IPDI system. Calorimetric measurement shows the crystallinity of the soft segment decreases in both the systems which results in decreasing shape fixity efficiency with increasing HSC.


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INTRODUCTION Recently, shape memory polymers (SMP) have attracted great attention as smart materials because of their attractive properties like light weight, large strain deformation (about 400500%), easy processability, easily given any arbitrary shapes and their potential applications in different fields viz. aerospace engineering, textile engineering, smart actuators and in intelligent biomedical devices.1–3 Generally, shape memory polymers have the ability to acquire a stable

shape temporarily and regain its permanent shape in presence of some stimuli viz. heat,4,5 light,6 electric field,7 water,8 magnetic field,9 solvent,10 and pH11,12 etc. After the first introduction of shape memory behavior in polynorborane type by Nippon Zion company in 1984, many polymers are reported as SMPs including poly(isoprene–butadiene–styrene), polyurethane and polystyrene series.13–16 Most of the SMPs are thermo-responsive i.e. their deformation or recovery of shapes takes place above the shape giving temperature (Thigh). Shape giving is a phenomenon through which a temporary shape is provided above a particular temperature, shape giving temperature (Thigh) and then fixed the shape at shape fixing temperature (Tlow). The shape giving temperature is usually slightly above the glass transition temperature (Tg) or melting temperature (Tm) of the polymer.17,18 In general, SMPs have cross-linked structure and the crosslinking may be either chemical or physical in nature.19,20 Self-assembly in polymeric system is responsible for superior mechanical, thermal properties and controlled drug delivery.21–23 Shape memory polyurethanes (SMPUs) have advantages over other SMPs because of its tunable properties24 that can be modified by changing the molecular weight, types of hard or soft segments or their ratio. First, SMPU was introduced by the Nagoya Research and Development Center of Mitsubishi Company in 1988,25 capable to recovery the permanent shape in the temperature range of -30 to 65 °C. Hard and soft segments are located in an alternative manner in 3 ACS Paragon Plus Environment


thermoplastic polyurethanes17 and the thermodynamic incompatibility between hard and soft segments generates phase separated structure. Usually, hard and soft segments of shape memory polyurethanes (SMPU) have different roles in showing the shape memory effect. The soft segment acts as a reversible actuating phase, the melting and crystallization of this phase helps in fixing temporary shape and recovering permanent shapes while hard segment helps to provides structural stability and rubber elasticity for shape recovery.14,26,27 The reversible actuating phases are oriented during deformation at shape giving temperature by the application of external force resulting an entropically unstable state. The soft segment usually made of polyether or polyester microglycol and the hard segment is made by the reaction of a diisocyanate and a low molecular weight diol.28,29 Polyurethane has other advantages like easily processable, resistance to organic and aqueous solvent, biocompatible and sometime biodegradable in nature. PCL in polyurethane, as soft segment, has the ability to improve biocompatibility, biodegradation along with better shape memory behavior.30 Wide range of research is carried out on SMPU after the first introduction of shape memory effect in polyurethane.31 A series of SMPU is synthesized by Kim et al. having crystalline reversible domain and they have made a thorough investigation on the effect of soft segment chain length on shape memory effect.14 Further, the effect of hard segment content on shape memory effect is investigated by Lin et al by synthesizing a series of polyurethane with amorphous reversible phase.32 The effect of morphology of segmented polyurethane on the shape memory effect has been reported showing better shape fixity and shape recovery at an optimum hard segment content (HSC) (20%) followed by decreasing tendency at higher HSC.33 Thermoplastic and thermoset SMPUs have been used in self-healing leather and paint,34 in transparent lens and containers,35 heat insulating foams,36 in component of


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woven and non-woven fabrics37 and in the biomedical field38 as self-expandable stent,39 selfknotting suture26 etc. In this work, we report different polyurethanes using a linear (HMDI) and an alicyclic (IPDI) diisocyanate and varying hard segment content. The preparation of different PUs and their hard segment content are confirmed through 1H-NMR and 13C-NMR measurement. The extent of interaction, through hydrogen-bonding, is compared and correlated with their structural details including varying hard segment content. The alteration of self-assembly in terms of structure has been outlined. The crystallization behavior of two opposite systems has been studied and its influence on mechanical and thermal properties is correlated. Shape memory behavior of both the systems has been compared and structure property correlations are drawn from the widest possible structural changes with their possible applications.

EXPERIMENTAL Materials: Poly(caprolactone diol) (PCL-diol) (Mn = 2000 g mol-1), 1, 6-hexamethylene diisocyanate (HMDI), Isophorone diisocayanate (IPDI) from Sigma-Aldrich, USA and 1,4butane diol (BD) (Meark, Germany) were used as received. The dibutyl tindilaurate (DBTDL) catalyst and the dimethyl formamide (DMF) solvent were purchased from Himedia and Merck, respectively. Synthesis of Polyurethanes: Different polyurethanes were synthesized in a two-step polymerization process. Initially, isocyanate terminated prepolymer was produced after the addition of excess diisocyanate in molten polyol. Successive reaction of this prepolymer with chain extender (butane diol) in the next step produces a multiblock copolymer of the segmented polyurethane. Polyurethanes with different hard segment content (HSC) were prepared by 5 ACS Paragon Plus Environment


changing the molar ratio of diisocyanate: polyol: chain extender as shown in Table 1. In the 1st step, diisocyanate (HMDI/IPDI) was mixed with PCL-diol at constant temperature of 70 °C under nitrogen atmosphere for 3 hour to prepare –NCO terminated prepolymer in a three neck round bottom flask. In the second step, chain extension was carried out with 1,4-butane diol in presence of catalyst (dibutyl tin dilaurate) for 24 hours to complete the polymerization reaction. The synthesis process is shown in Scheme 1. The polymer was collected by pouring the solution in deionized water followed by drying in vacuum at 60 °C under reduced pressure for 48 hours. The molecular weights of the synthesized polyurethanes were measured through gel permeation chromatography (GPC), summarized in Table 1. Characterization. Spectroscopic Measurement. Proton NMR spectra were recorded on a Bruker spectrometer after dissolving the synthesized samples in d6-DMSO solvent. Samples were equilibrated in the magnetic field for 10 min before recording the spectrum. The chemical shifts are reported in ppm unit relative to tetramethylsilane (TMS). Infrared spectra of solid polyurethane films were recorded on a Thermo Nicolet 5700 Fourier transform IR (FTIR) taking 100 scans with the resolution of 4 cm-1 at room temperature. UV-Vis measurement was done with Jasco V-650 spectrophotometer in the spectral range of 200-800 nm taking solid thin film of the synthesized samples. Each spectroscopic measurement was done in triplicate to obtain better precision. Thermal Stability. The thermal stability of the specimens was estimated through Mettler thermogravimetric analyzer (TGA) in the temperature range of 40-600 °C at a heating rate of 20o min-1 under N2 atmosphere. The melting, crystallization temperature and heat of fusion of different synthesized polyurethane samples were recorded using Metler 832 differential scanning


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calorimetry (DSC) over a temperature range of -40 to 200 °C. The scanning rates were 10o/min and 5o/min during heating and cooling, respectively. The DSC was calibrated with Indium before use. To get better error estimation, the measurements were repeated thrice. Mechanical Responses. The elongation at break, tensile strength, modulus and toughness of HMDI and IPDI based PUs were measured using Instron 3369 tensile tester at room temperature, maintaining the strain rate of 5 mm/ min. Samples were prepared through solvent cast technique keeping the dimension of the samples as 0.5×10×50 mm3. For obtaining better error estimation, experiment was carried out in triplicate. Morphological Investigation. The morphology of the different synthesized polyurethanes (PUs) was examined using atomic force microscope (AFM) and polarizing electron microscope (POM). For AFM measurement NT-MDT multimode AFM, Russia, was used attached with Solver scanning probe microscope controller. Tapping mode was used and the tip was mounted over 100 µm long, single beam cantilever with resonant frequency range of 240-255 Hz and the associated spring constant of 11.5 N/m. The bulk morphology of the thin films of the synthesized PUs was investigated with the help of polarizing optical microscope (POM). Morphological investigations were performed with three different samples and at different places to eliminate the artifacts. Structural Analysis. X-ray diffraction of the synthesized polyurethanes was investigated using Rigaku Miniflex wide angle X-ray diffractometer taking Cu Kα source of 0.154 nm wavelength and maintaining the voltage and current of the generator at 40 kV and 15 mA. Samples were placed on quartz holder at room temperature and were scanned at diffraction angle (2θ) range of 2 to 40° keeping the scanning rate of 3°/ min. Small angle neutron scattering (SANS) measurement was executed with the spectrometer placed at Dhruva reactor of Bhabha



Atomic Research Center, Mumbai, India. The scattering from the synthesized samples were improved for background influence and data were collected in the scattering vector range (q) of 0.17 to 3.5 nm-1. The lower range data of scattering vector was fitted with different models viz. Debye-Bueche model for the calculation of correlation length (ξ). The temperature was kept at 30 °C throughout the measurement. Method of Shape Memory Effect. The Shape memory behaviour of the synthesized polyurethanes were verified using straight strips of dimension 60×3×0.5 mm3. At first, strips were deformed into ring like shapes at shape giving temperature (Thigh) followed by fixing the deformed shapes at shape fixing temperature (Tlow). Recovery of the permanent shapes was observed on further heating the strips at 37 °C for IPDI based PUs and at 90 °C for HMDI based PUs. Shape memory behavior was quantified using universal tensile tester (Instron 3369) machine. The samples of 50×10×1 mm3 dimension was stretched to 100% keeping the strain rate of 5 mm / min at shape giving temperature (Thigh=90 °C) and was equilibrated for 5 min followed by the freezing the deformed sample at Tlow (5 °C). After removing the stress, freezed length of the samples was measured. Then, samples were heated at Thigh and samples start to recover their initial shapes. The process was repeated with three different samples to obtain more accurate results. The shape fixity ratio (Rf) and shape recovery ratio (Rr) were calculated using the equation 1 and 2 given below. Shape fixity ratio (Rf) = [(l3-l1)/l1]×100

(1)

Shape recovery ratio (Rr) = [(l2-l4) / l1]×100

(2)

where, l1 represents the initial length of the strip, l2 is the length after stretching at Thigh while l3 corresponds to length of the strip after stress released and l4 is the recovered length at Thigh.


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RESULTS AND DISCUSSION Spectroscopic Analysis of Varying Structure and Interactions. Different types of polyurethanes (PUs) are synthesized using two different diisocyanates (hexamethylene diisocyanate and isophorone diisocyanates) with varying hard segment content. The polyurethanes which are synthesized by varying HSC using HMDI, PCL-diol and BD are designated as H-10, H-30 and H-50, where the numbers represent the HSC percentages. Similarly, polyurethanes which are synthesized using IPDI, PCl-diol and BD are termed as I-20, I-30 and I-50 with similar meaning. The chemical structure of the synthesized PUs is determined using 1H-NMR and 13C-NMR. Figure 1a and 1b represents the 1H-NMR spectra of HMDI and IPDI based PUs, respectively. The features of the respective Figures are in good agreement with each component of polyurethane chains and assignments of the peaks are done following the literature reports.40–42 The peaks at around δ = 7 ppm is responsible for the urethane >N-H proton and its intensity increases with increasing hard segment content (HSC) for both types of polyurethanes (PUs) which is further supported by 13C-NMR spectra of both the PUs (Figure 1c & 1d). The relative intensity of the peak appears at δ =157 ppm increases with respect to the peak at δ =173 ppm with increasing hard segment content in

13

C-NMR spectra of both the

polymers. This is to mention that the peaks appear at 157 and 173 ppm in 13C-NMR spectra, are responsible for urethane and ester >C=O groups present in polyurethane hard and soft segments, respectively. Further, the percentage of hard segment content (HSC) is calculated from the integral area of the spectra matches well with the values calculated from weight taken during syntheses and are presented in Table 1. FTIR investigation of both the PUs (Figure 2a & 2b) shows that the intensity of the urethane carbonyl group (>C=O) appears at 1687 cm-1 for H-10 and 1700 cm-1 for I-20 shifts to lower wavenumber with increasing HSC having higher intensity, 9 ACS Paragon Plus Environment


which signifies the increase of urethane linkages in PU as well as the interaction between the PU chains through hydrogen bonding.43 Further, the shifting of >N-H stretching frequency also supports the same phenomenon (Figure S1a & S1b). The peak for HMDI-based PUs appears at 1687 cm-1 in H-10 for urethane (>C=O) group which shifts to 1678 cm-1 in H-50 while relatively small shifting from 1700 to 1697 cm-1 occurs in IPDI-based PUs. Further, the simulation of energy minimized configuration through AM1 method shows relatively small distance of ~2.07 Å between oxygen atom of ester carbonyl and hydrogen atom of amide >N-H group for HMDIbased PUs44 against ~5.9 Å distance for IPDI-based PUs of similar45 bond pair. The presence of bulkier cyclohexane ring in IPDI system restricts the interaction and increases the distance between the chains against strong hydrogen bonding in linear chains in HMDI-based PUs. Moreover, the appearance of UV absorption (Figure 2c & 2d) bands at 267 nm in H-10 and at 277 nm in I-20 are responsible for n−π* which shift to 280 nm in both H-50 and I-50 PUs. The greater red shifting in HMDI-based PUs further confirms higher extent of interaction. However, all three spectroscopic methods confirm the relative hard segments in PUs and compare the interactions between polymer chains indicating stronger interaction in HMDI based PU vis-à-vis IPDI based PU underlying their linear vs. allicyclic structure, responsible for varying interactions. Thermal and Structural Characteristics. Figure 3a and b show the TGA thermograms of HMDI and IPDI-based PUs, respectively, with varying HSC. The DTG plots are attached in the inset of the each Figure. The degradation temperature, obtained from DTG curves, are 373, 363, 350, 359, 354 and 352 °C for H-10, H-30, H-50, I-20, I-30 and I-50, respectively. Thermal stability decreases with increasing hard segment content for both type of PUs while the extent of decrease is more in HMDI-based PUs as compared to IPDI-based PU. The hard segment is more


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prone to heat and degrade first than that of soft segment and the greater association in high HSC HMDI based PUs is affected more towards thermal stability.46 The crystal structure of different PUs is determined through wide angle X-ray diffraction in the scattering angle of 10 to 40°. XRD patterns (Figure 3c) of HMDI-based PUs show Bragg’s reflection peaks at ~21.4°, 23.8°, 22.01° and 24.3° for the crystal planes of 0.42 nm (110), 0.377 nm (200), 0.41 nm (201/211) and 0.37 nm (010/210), respectively.47,48 Amongst them, the peaks at 21.4° and 23.8° correspond to the crystalline soft segment (PCL-diol) while 22.01° and 24.3° peaks are assigned to the crystalline hard segment. Interestingly, the intensity of the peaks related to soft segment decreases and the intensity of the peaks corresponds to hard segment increases with increasing HSC content. So, the greater interactions, through hydrogen bonding, between the urethane linkages of the adjacent polyurethane chains help to crystallize the hard segment in greater extent. The diffraction pattern of IPDI-based PUs are shown in Figure 3d showing Bragg’s reflections only for crystalline soft segment at 2θ ~ 21.4° and 23.8°. Further, the intensity of the peaks decreases with increasing HSC content. Hence, the crystallinity of the IPDI-based PUs decreases with increasing HSC and it becomes amorphous when percentage HSC has increased to 70% (Figure S2). This is to mention that IPDI-based PUs does not possess any peak corresponding to the crystalline hard segment. However, the peak appears at 13.5° in HMDI-based PUs corresponds to crystal plane (002), whose intensity increases with increasing HSC and it is categorically absent in IPDI-based PUs. The (002) plane corresponds to the stacking of the polymeric chain under the influence of extensive hydrogen bonding. The linear chain in the hard segment facilitates the staking of the hard segment while the presence of bulkier alicyclic ring in the hard segment of IPDI-based PUs restricts the stacking of the hard segment (cf. FTIR measurement).



Mechanical Responses. The mechanical strength of the synthesized PUs is measured using Universal Testing Machine (UTM). The stress-strain curves of HMDI-based PUs are shown in Figure 4a. The elongation at break decreases with increasing HSC. H-10 shows~ 420% elongation at break against only~110% elongation at break is observed in H-50. Interestingly, a reverse phenomenon takes place in case of IPDI-based PUs. I-50 shows ~100% elongation at break whereas meager ~5% elongation is shown in I-20 (Figure 4b). However, stress-strain curves of IPDI-based PUs exhibit higher elongation at break with increasing HSC. The tensile modulus and toughness of both HMDI-based and IPDI-based PUs are calculated after fitting the initial data points of stress strain carve and from the integration area under the stressstrain curve. Figure 4c shows the decreasing toughness, from 9.4 MJ.m-3 in H-10 to 6.2 MJ.m-3 in H-50, in HMDI-based PUs while modulus decreases slightly from 100 MPa in H-10 to 90 MPa in H-50. Usually, tensile modulus of polyurethanes increase with increasing hard segment content49 due to higher crystallinity of hard segment but in our developed HMDI-system slight reduction is noticed. The tensile modulus and toughness of IPDI-based PUs are shown in Figure 4d. Young’s modulus of I-20, I-30 and I-50 are 118, 102 and 54 MPa, respectively, showing consistent decreasing order with increasing HSC predominantly because of downward tendency of crystallinity. On contrary, higher toughness is observed from 0.14 to 1 MJ.m-3 when hard segment content increase from 20 to 50% in IPDI-based PUs. The increment in toughness with HSC in IPDI-based PUs against lowering of toughness with HSC in HMDI-based PUs is justified from the lowering of crystallinity or more amorphous nature of IPDI system in absence of crystalline hard segment which assist the polymer chains to slip over each other while greater crystalline hard segment in higher HSC HMDI system increase the chances of brittle fracture.50


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Understanding Self-Assembly in Different Systems. A comparative study of molecular self-assembly of polymeric sheets of HMDI and IPDI system is studied through finding the smallest distance between two molecular sheets which is obtained from XRD while the characteristic length (Λc), obtained from SANS, provides knowledge about the minimum number of molecular sheets required to form a lamellae. The extensive H-bonding between molecular sheets leads to an assembly of polymeric chains along (001) plane.51 HMDI-system shows XRD peak at 2θ~5.7° corresponding to the d-spacing of 1.5 nm (Figure 5a(i)). The presence of this reflection indicates that polymeric chains of HMDI-based PUs stacked along the (001) plane through extensive hydrogen bonding (cf. as evident from FTIR investigation). Further, small angle neutron scattering (SANS) measurement (Figure 5a(ii)) show hump at a scattering vector of 0.33 nm-1 for H-30, corresponding to characteristic length (Λc= 2π/qm, where, qm is the wavevector associated with the peak position) ~ 19 nm which shifts to 0.40 nm-1 (Λc~ 13 nm) in H-50 indicating smaller stack requirement for higher HSC content samples. Hence, the decrease of characteristic length from 19 nm to 15 nm indicates greater close packing at higher HSC and lesser number of molecular sheets are required to form lamellae. The blob size (Figure S3a) obtained after correlating the initial q values with Debye-Bueche equation increases first from 1.3 nm for H-10 to 2 nm for H-30 followed by little decrease to 1.7 nm in H-50. The bottom up self-assembly develops domain like structure and become observable in AFM. For HMDIsystem, the domain sizes are 410, 500 and 450 nm for H-10, H30 and H-50, respectively (Figure 5a(iii)). These domains further assemble to develop cluster and is observed with polarizing optical microscope (POM) showing cluster sizes of 7, 4 and 4.2 µm for H-10, H-30 and H-50, respectively (Figure 5a(iv)). On the other hand, IPDI systems show XRD peak at 2θ ~ 4.3°, corresponding to d-spacing of 2.1 nm for I-20 (Figure 5b(i)) which disappear with increasing 13 ACS Paragon Plus Environment


HSC and no definite peak is appeared in SANS patterns of IPDI-systems (Figure 4b(ii)) which suggests that weak interchain interaction leads to weaken the assemble the molecular sheets with increasing HSC. The blob size (ξ) for IPDI-system increases from 1.3 nm to 1.9 nm with increasing HSC (Figure S3b). Again, the morphological investigation with atomic force microscope shows that the domain size increases from 261 to 320 nm with the increase of hard segment content (Figure 5b(iii)). Further, the observation with polarizing optical microscope (Figure 5b(iv)) reveals that cluster size increases from 3.5 to 4.2 µm for I-20 and I-30, respectively. Hence, the extensive hydrogen bonding in HMDI-system as compared to weekly interacted assembly in IPDI-system is responsible for varying clusters in two different systems based on the extent of hydrogen bond formation, which in turn, arise from the linear vs. alicyclic structure of PUs. These unique and varying self-assembly should affect the shape memory behavior of two very different PU. Shape Memory Behavior as a Function of HSC. Shape memory process comprises of deformation of a stable permanent shape above a transition temperature, shape fixing via fast cooling and recovery of the permanent shape above the transition temperature. The melting point of soft segment of thermoplastic polyurethane acts as the transition temperature and reversible melting and crystallization of soft segment above and below the transition temperature to control the shape memory process.15 Figure 6a and b are the representative photographic images of shape memory behaviors of HMDI and IPDI-based PUs with varying HSC. Shape memory efficiency of the synthesized PUs is verified in terms of shape fixing (Rf) and shape recovery (Rr) ratio after deforming the straight strips of different types of PUs into ‘ring’ shape at shape giving temperature (Thigh) followed by fixing the deformed shapes at shape fixing temperature (Tlow). This is to mention that the shape giving temperature is different for HMDI and IPDI systems and


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they are chosen in order to obtain maximum shape memory efficiency. For HMDI system, the shape giving temperature (Thigh) is 90 oC while for IPDI system it is 37 °C although the shape fixating temperature is same for both the system (5 °C). The recovery of permanent shape takes place when the deformed rings are placed at Thigh. It is observed that a bent shape is noticed after recovery in H-10 PU while a straight shape is obtained for H-30 and H-50 PU when they are placed at 90 °C hot water bath. Further, a complete ring shape is formed for H-10 during shape fixing against only half ring shape is taken place in H-50 which suggest that shape fixity ability decreases with increasing hard segment content in HMDI-based PUs. On the other hand, when deformed strips (‘ring’ shape) of IPDI-based PUs are placed in 37 °C hot water bath, recovery of permanent shape takes place leading to almost straight strip for I-30 while curved shape is observed in I-50 PU. The shape fixity was made at 5 °C (Tlow). Hence, shape recovery increases for HMDI system while decreases in IPDI system with increasing HSC content. The quantification of shape fixity and shape recovery of HMDI-based PUs is done after deforming the samples uniaxially up to 100% followed by freezing the deformed specimens at 5 °C (Figure S4 (a,b,c)). Figure 6c shows that shape fixity for H-10, H-30 and H-50 are 98, 94 and 65% and shape recoveries are 70, 91 and 96%, respectively. So, shape fixity decreases while shape recovery increases with increasing HSC content for HMDI-based PUs. The thermomechanical cycles of H-30 and H-50 show that shape fixity and recovery ratio decreases with number of cycles. This phenomenon is justified from the fatigue behavior of the materials (Figure S5a,b). The increasing assembly of the hard segment with increasing HSC generates more retractive force which helps the polymeric strips to regain their permanent shape. But in IPDI system, shape recovery first increases from 87% in I-20 to 90% in I-30 then it decreases to 79% in I-50 while shape fixity continuously decreases with increasing HSC and the shape fixity ratio of I-20,



I-30 and I-50 are 98, 97 and 95%, respectively (Figure 6d). Ji et.al reported decrease in shape fixity ratio in MDI based PU with increasing HSC content.33 This is to mention that shape fixity and recovery of IPDI-based PUs are calculated by measuring the end to end distance of the strip before shape giving, after shape giving, shape fixing and after shape recovery. Lin et al. reported that shape recovery ratio decreases with decreasing modulus which justify the shape recovery obtained in IPDI system but contrary to the result obtained in HMDI-system. However, polyurethanes with 30% hard segment content shows better shape memory behavior in terms of shape fixity and shape recovery in both IPDI and HMDI-based PUs. To understand different shape memory behaviors in different PUs, differential scanning calorimetric (DSC) measurement is performed. The DSC thermograms show the heating-cooling cycles of HMDI and IPDI-based PUs related to melting and crystallization behavior of the synthesized polyurethanes (Figure 7a(i,ii,iii) and 7b(i,ii,iii)). The reversible melting and crystallization of soft segment are the important factor for controlling the shape memory behavior. The melting temperatures and heat of fusion (∆H) values of soft segment decreases during 1st heating from 51 °C (25 J.g-1) in H-10 to 33 °C (18 J.g-1) in H-30 and the crystallization temperature during 1st cooling decreases from 18 °C in H-10 to 3 °C in H-30 for soft segment. The melting and heat of fusion of soft segment during 2nd heating and cooling in HMDI system are 48 (21 J.g-1) and 34 (10 J.g-1) for H-10 and H-30, respectively, while the melting temperature of hard segment changes from 128 °C in H-30 to 172 °C in H-50. The crystallization temperature during second heating changes from 11° to 9 °C for H-10 and H-30, respectively. This is to mention that H-10 and H-50 does not show any definite melting temperature for hard and soft segment, respectively. The increase in melting temperature of hard segment is due to increased stacking of the hard segment under the influence of strong hydrogen bonding for HSC PUs (as


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evident from FT-IR). Further, the interaction between hard and soft segment lowers the melting temperature of the soft segment with increasing HSC.52 The higher crystallization temperature of soft segment in H-10 leads to better shape fixity value followed by H-30 and least shape fixity in H-50 is due to decreasing crystallization behavior of soft segment which is also varied through changing of morphologies with heating-cooling cycles (Figure S6). However, decrease in melting temperature of soft segment along with increasing stacking of the hard segment help to increase the shape recovery ratio with increasing HSC in HMDI system. In IPDI system, the melting temperature during 1st heating are 37, 51 and 51 °C, for I-20, I-30 and I-50, respectively, while during 2nd heating only I-20 has the melting temperature at 44 °C presumably because only soft segment of I-20 is able to crystallize during heating. There is no melting point found for hard segment in IPDI system. Hence, hard segment of IPDI system fails to crystallize due to presence of bulky alicyclic ring in the main chain, as evident from XRD measurement also, where no characteristic crystalline plane of hard segment is observed (Figure 3d). The lower melting temperature and ability to crystallize help to increase shape fixity ratio of I-20 as compared to others. However, decreasing crystallization ability with HSC leads to poor shape fixity ratio from I-20 to I-50 PUs.14 Further, increasing melting temperature of soft segment leads to incomplete melting of soft segment at Thigh (37 °C) and also the absence of crystalline hard segment leads to lowering of shape recovery efficiency with increasing hard segment content in IPDI-based PUs. Further, the nanostructure investigation (XRD measurement) shows that the linear structure of the HMDI system facilitates consolidation of the hard segment along (001) plane through hydrogen bonding within the molecular sheets. Hydrogen bonding takes place between >C=O groups of ester (soft segment) and >N-H group of urethane linkages (hard segment) which increases with increasing hard segment content (HSC) and results higher stacking density (SANS



measurement) along with the greater distribution of hard segment within soft segment which decreases the melting temperature of soft segment. On the other hand, a reverse phenomenon takes place in bulky alicyclic IPDI system, where a weak consolidation occurs in lower HSC which gradually disappear with increasing the HSC (as evident from XRD and SANS measurement). Therefore, a differential nanostructure is observed in HMDI and IPDI system. This difference leads to reverse results in shape memory behavior. It is observed that with increasing HSC in HMDI system shape recovery increases while IPDI system exhibits opposite behavior in shape recovery (decreases with increasing HSC). Hence, the increasing consolidation of the nanostructure with increasing HSC generates more retractive force in HMDI system which results better shape memory behavior. On contrary, decreasing consolidation in IPDI system with increasing HSC fails to generate retractive force and results in limited shape recovery. Hence, the shape memory behavior in polyurethane is controlled by its nanostructure. However, the effect of segment and its content has been established for ultimate shape memory behavior in polyurethane keeping the other structural unit intact. Thus, shape memory effect is regulated by using different chemical structure (linear or alicyclic) in the main polymer chain and the total hard segment present in the polyurethane chains. A ‘spring model’ is proposed to explain the differential shape memory effects of HMDI and IPDI systems considering the experimental findings, as shown in Figure 7c. In this model, the soft segment is acts like spring and two ends of the spring is hold by the consolidated hard segment and some hard segments are distributed in soft segment due to hydrogen bonding between ester >C=O and urethane >N-H group. The linear structure of the HMDI system facilitates extensive hydrogen bonding with both ester and urethane carbonyl groups which lead to better consolidation along with distributed hard segment within soft segments which


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eventually increases with increasing HSC and results greater shape recovery. On the other hand, week interaction due to bulky structure of IPDI system leads to less consolidation which further decreases with increasing HSC and result in decreasing shape recovery at higher HSC. However, a different grade of shape memory effect is observed in two different types of polyurethane and structure can control the shape memory behavior of polymers.

CONCLUSION Various polyurethanes using linear and alicyclic diisocyanate have been synthesized with varying degree of hard segment content. Structure and quantification of hard segment has been measured through NMR spectroscopy. The interaction between polymer chains as a function of hard segment has been revealed through FTIR and UV measurements. The linear structure of the HMDI systems facilitates interchain interaction more effectively as compared to bulky structure of the IPDI systems. XRD measurements show higher degree of hard segment crystallinity and lower crystallinity of soft segment in high HSC PUs. Extensive hydrogen bonding in hard segment, as revealed by FTIR and other measurements, is responsible for varying crystallinity and bulkier alicyclic ring in IPDI system prohibit the formation of hydrogen bonding and thereafter the overall crystallinity. HMDI system generates crystalline hard segment and crystallinity increases with increasing HSC in contrary week interaction in IPDI system fails to crystallize the hard segment which leads to amorphization with increasing HSC. The greater interaction/crystallinity limits the slipping of the polymer chains in HMDI system and lowers the toughness with increasing HSC while decreasing crystallinity of the IPDI system increases the toughness with increasing HSC. Shape memory efficiency has been worked out for various PUs. Shape recovery increases while shape fixity decrease with HSC content in HMDI-based PU. On 19 ACS Paragon Plus Environment


the other hand, slight decreased shape recovery is observed in IPDI system at higher HSC keeping the shape fixity almost similar. Layer by layer self-assembly has been revealed through XRD, SANS, AFM and optical images showing greater inhomogeneities in HMDI-based PUs against considerably lower values measured in IPDI system. The increasing self-assembly in HMDI system generates more retractive forces which lead to greater shape recovery value while recovery value increases initially followed by its decrease at higher HSC. Thus, shape memory effect can be regulated by using appropriate hard segment and chemical structure of polyurethane.

Acknowledgement: The authors acknowledge central instrument facility of IIT (BHU) for AFM, NMR measurements. The authors also want to acknowledge the Science and Engineering Research Board (Grant R&D/SERB/LT/SMST/ 16/17/06) (SERB), New Delhi for financial support.

Supporting Information Available: The supplementary Figure S1 describes FTIR investigation of the synthesized PUs. Supplementary Figure S2 is about the structure of IPDI based PU with HSC 70%. Supplementary figure S3 is the fitting of the ‘q’ values of SANS with Debye-Bueche model. Supplementary Figure S4 demonstrates the quantification of shape memory effect using UTM. Supplementary Figure S5 describes the thermomechanical cycles of PUs. Supplementary Figure S6 shows the changes of morphologies of PUs with temperature. The supporting information is available free of charge via the Internet at http://pubs.acs.org.

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Table 1: Molar Ratio, Molecular Weight and HSC Contents of the Synthesized Polyurethanes Tabulated

PUs

#

Molar Ratio

Molecular Weight# HSC content HSC content (theoretical) (experimental)* M M n

w

H-10

2:1.6:0.4

40,860

64,700

10.4

10.9

H-30

3.7:1:2.7

35,680

64,000

30.2

31.1

H-50

8:1:7

45,900

75,000

49.7

49.5

I-20

1.9:1:0.9

38,852

58,418

19.4

19.6

I-30

3:1:2

36,252

29.8

30.2

I-50

6.8:1:5.8

49,847

50.4

49.9

56,447

68,947

Polystyrene is taken as standard; *HSC is calculated from H-NMR data.



Scheme 1: Synthesis of different polyurethanes using two different diisocyanate (HMDI and IPDI) and varying hard segment content.


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Figure 1: The 1H-NMR spectra of a)

HMDI-based

polyurethanes and

b)

IPDI-based

polyurethanes with increasing hard segment content. The spectra were taken after dissolving the samples in d6-DMSO solvent. The intensity of the peak at δ value ~7 ppm corresponds to >N-H hydrogen increases with increasing HSC. The 13C-NMR spectra of c) HMDI-based and d) IPDIbased polyurethanes with increasing hard segment content. The peaks appears at 173 and 157 ppm are responsible for the >C=O carbon of ester and urethane group, respectively.



b)

H-10

Reflectance / a.u.

Reflectance / a.u.

a)

1687

H-30

1681

H-50

I-20 1700

I-30 I-50 1697

1678

2000

1500

2000

1000

1500

Wavenumber / cm

H-50 H-30 H-10

279 nm

d) Absorbance

c)

267 nm

300

400

500

1000

Wavenumber / cm-1

-1

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 36

600

280 nm

I-50 I-30 I-20

277 nm

300

Wavelength / nm

400

500

600

Wavelength / nm

Figure 2: The FT-IR investigation of a) HMDI-based PUs and b) IPDI-based PUs shows that the relative intensity of the peak for amide >C=O group increases with increasing hard segment contents of the PUs. Further, the extensive H-bonding shifts the stretching frequency of amide >C=O group to lower wavenumber in HMDI system compare to IPDI system. The UV-vis spectra of c) HMDI system and d) IPDI system shows that the absorbance value due to n-π* transition shifts to higher wavelength with increasing HSC which indicates greater extent of interaction takes place with increasing HSC.


H-10 H-50 H-30 o

363 C o

o

373 C

350 C

300

400

500

0.5

I-20 I-30 I-50

I-20 I-30 I-50 o

352 C

200

300

300 T/ C

Intensity / a.u.

(110)

d)

(200) (010/210)

(110)

300

500

T/ C

H-30 H-50

10

500

o

(002)

H-10

0.0 100

500

o

c)

o

400

T/ C

o

T/ C

0.0 100

o

359 C

o

354 C

(200)

200

Weight fraction

0.5

b)1.0

H-10 H-30 H-50 Heat flow / a.u. (exo up)

Weight fraction

a) 1.0

Intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Heat flow / a.u. (exo up)

Page 31 of 36

I-20 I-30 I-50

20

10

30

20

30

2θ / deg

2θ / deg

Figure 3: The thermal stability of a) HMDI system and b) IPDI system is expressed interms of weight fraction remain with increasing temperature.

The XRD measurement shows the

structural change of both c) HMDI and d) IPDI systems with increasing HSC. The (110) and (200) crystal planes are responsible for crystalline soft segment while 010/ 210 plane is responsible for crystalline hard segment. In both the systems, the intensity of the (110) and (200) planes decreases while the intensity of the (010/210) plane increases with increasing HSC.



c)

0

0

200

H

-5 0 H

-5 0

0

H

H

H

-3 0

-1 0

0

H-10

5

-1 0

3

50

10

H

Modulus / MPa

100

-3 0

Toughness / MJm

H-50 H-30

6

σ / MPa

-3

a)

400

ε/% d)

60

I50

0.0

30

0.5

I-

I50

I30

0

I-50

0 0

50

I10

2

1.0

20

100

I-

Toughness / MJm

-3

I-20 I-30

Modulus / MPa

/ MPa

b)4

σ

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 36

120

ε/%

Figure 4: The mechanical properties of a) HMDI system and b) IPDI system is measured using

UTM. The elongation at break decreases in HMDI system while increases in IPDI system with increasing HSC. The modulus and toughness of c) HMDI system and d) IPDI system are calculated after fitting the initial values and taking the area under the carve. The modulus in HMDI-system decreases from H-10 to H-30 then further increases in H-50 while in IPDI system it decreases continuously and toughness in HMDI system continuously decreases while in IPDI system continuously increases.


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Figure 5: The step by step self-assembly of a) HMDI system and b) IPDI system is determined

using XRD (i), SANS (ii), AFM (iii) and POM (iv). In HMDI-system the assembly of the polymer chain increases with increasing hard segment content as evident from increasing intensity of the 001 plane at 2θ ~5.8o which reduces the characteristic length and increases the blob size while reverse phenomena takes place in IPDI system where intensity of the 001 plane at 2θ~4.3o disappear with increasing HSC which results an amorphous morphology with increasing HSC. The bar represents 100 µm for a(iv) and 50 µm for b(iv).



Figure 6: The representative photographic images of shape memory behavior of a) HMDI-based

and b) IPDI-based PUs. Straight strips of the samples are deformed at Thigh (90 oC for HMDI system & 37 oC for IPDI system) and deformed shapes are fixed at Tlow (5 oC). Further, the regain of the permanent shapes take place on heating at Thigh; c) The shape fixing and recovery ratio of the c) HMDI system and d) IPDI system with increasing HSC is shown here. The fixity ratio decreases in both the cases and the recovery ratio increases with increasing HSC for HMDI system while in IPDI system recovery ratio increases upto I-30 then decreases in I-50.


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Figure 7: Differential scanning calorimetric investigation of a) HMDI-based PUs and b) IPDI-

based PUs. The scanning was done in the temperature range of -40 to 200 oC keeping scanning rate 10o min-1 during heating and 5o min-1 during cooling; c) The spring model is proposed for demonstrating the shape memory effect of HMDI and IPDI system where soft segments act like spring which is sandwich between hard segment. The strong end portion of HMDI based PUs is responsible for its better shape memory behavior.



TOC IMAGE


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Nanostructure Controlled Shape Memory Effect in Polyurethanes

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