Surface characteristics of chitin-based shape memory polyurethane elastomers

Colloids and Surfaces B: Biointerfaces 72 (2009) 248–252

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Colloids and Surfaces B: Biointerfaces journal homepage: www.elsevier.com/locate/colsurfb

Surface characteristics of chitin-based shape memory polyurethane elastomers Khalid Mahmood Zia a , Mohammad Zuber b,∗ , Mehdi Barikani c , Ijaz Ahmad Bhatti a , Mohammad Bilal Khan d a

Department of Chemistry, University of Agriculture, Faisalabad 38040, Pakistan Department of Industrial Chemistry, Government College University, Faisalabad, Pakistan c Iran Polymers and Petrochemicals Institute, P.O. Box 14965/115, Tehran, Iran d School of Chemical and Material Engineering, National University of Science and Technology, Islamabad, Pakistan b

a r t i c l e

i n f o

Article history: Received 15 December 2008 Received in revised form 5 April 2009 Accepted 8 April 2009 Available online 16 April 2009 Keywords: Chitin Shape memory polyurethane Surface free energy Contact angle Dimethylol propionic acid

a b s t r a c t Shape memory polyurethanes (SMPUs) were prepared from polycaprolactone diol 4000 (PCL 4000), 1,4-butanediol (BDO), chitin, dimethylol propionic acid (DMPA), triethylamine (TEA) and 4,4 diphenylmethane diisocyanate (MDI), and the structures of the synthesized materials were verified by infrared spectroscopy. The effects of chitin and DMPA contents in the polyurethane formulation on surface properties were investigated. DMPA provides function of making hydrophilic polyurethanes. The crystalline structure of chitin enhanced the hydrophobicity of the synthesized materials. Contact angle, water absorption, surface free energy, work of water adhesion and swelling behavior of the synthesized polyurethanes were affected by varying the DMPA and chitin contents. The interactions of the PU films with solvents on the surface were clearly related to the contents of DMPA and chitin in the final polyurethane formulation. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Shape memory refers to the ability of materials to remember their original shape even after severe deformation. During the past few decades shape memory polymers are gaining more and more attention because of their low cost, low density, high shape recoverability and easy processability [1–3]. Shape memory polymers (SMPs) are polymer networks equipped with suitable molecular switches, which are sensitive to an external stimulus. Polymer networks consist of chain segments and net points. The chains segments are crosslinked at net points determine the permanent shape of the polymer. The crosslinks can be either of a chemical nature (covalent bonding) or of a physical nature (intermolecular interactions). Among various SMPs, shape memory polyurethanes (SMPUs) are receiving much attention for their easy control of glass transition temperature (Tg) around the room temperature and excellent shape memory properties at the room temperature. Shape memory polyurethane (SMPU), a novel class of functional materials, has been extensively researched since its discovery by Mitsubishi in 1988 and attracting a great deal of attention recently, due to their unique properties [4–8]. The SMPUs basically consist of two phases, the frozen phase and the reversible phase. Hard segments in polyurethanes can be formed via hydrogen bonding and

∗ Corresponding author. Tel.: +92 321 6682375; fax: +92 41 9230098. E-mail address: [email protected] (M. Zuber). 0927-7765/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfb.2009.04.011

crystallization, acting as frozen phase below the melting temperature. The transformation of the soft segment acting as the reversible phase is responsible for the shape memory effect [6]. This shape memory effect can be controlled via the molar ratio of the hard and soft segments, the molecular weight of the soft segment, and the polymerization process. A series of SMPUs from polycaprolactone diol, 1,4-butanediol (BDO), dimethylol propionic acid (DMPA), and 4,4 -diphenylmethane diisocyanate (MDI) or toluene diisocyanante (TDI) were synthesized [5,9–15]. Up to the present, most SMPUs are prepared from linear polyurethane, which have physically crosslinked segments. In order to fulfill the various needs regarding the physical properties and enhancement of the shape memory function, in the previous paper [16], we had characterized chitin-based polyurethane with pronounced shape memory effect and thermo-mechanical properties by introducing a series of shape memory polyurethanes. Some of these polymers were crosslinked by introducing chitin in its structure. Extensive work on detailed molecular characterization [17], XRD studies [18], and thermal properties [19] of chitin-based polyurethane elastomers (PUEs) have also been previously discussed and reported. In vitro biocompatibility and non-toxicity of chitin/1,4-butanediol blends based polyurethane elastomers has also been reported elsewhere [20]. Surface morphology of starch [21], cellulose [22,23] and chitin–humic acid [24] has also been investigated and well documented. No reference is still available concerning the study of surface properties of chitin-based shape memory polyurethane.

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The structure of chitin composed of sugar and amino acid residue is analogs of protein and growth factor in the human body which is more relevant for stimulating the appropriate physiological response required for cellular regeneration and tissue restructuring in wounds. Moreover, chitin is composed of Nacetyl-d-glucosamine monomers which occur in hyaluronic acid, important in wound repair. Therefore chitin may possess the characteristics favorable for promoting rapid dermal regeneration, accelerating wound healing which is one of the most important feature of biomedical materials [25]. Considering versatile properties of PU and wound healing properties of the chitin, it is expected that the resulting product will be a suitable candidate for biomedical implants. In the present work a systematical explanation based on study of the effect of chitin and DMPA contents in polyurethane structure on the surface characteristics of PUEs has been provided.

DSA 10 goniometer of Kruss GmbH (Germany), equipped with software for a drop shape analysis. Diiodomethane (analytically pure, UCB, Belgium) and deionized water were applied as the test liquids. The image of liquid drop (volume of 2–3 ␮l) was recorded by video camera and fitted by means of mathematical functions. Each given  value was the average of at least 10 measurements and the precision was 1◦ . Surface free energies ( s ) and their dispersive (sd ) and p polar (s ) components were calculated by Owens–Wendt method [28] and Wu method [29]. From  measurement, the work of water adhesion to PU surface was also obtained. Water absorption (%) and equilibrium degree of swelling were determined following the method as reported elsewhere [30]. All the data presented were average of seven measurements.

2. Experimental

Shape memory polyurethanes (SMPUs) were prepared from PCL 4000, MDI, BDO, DMPA, TEA and chitin. PCL 4000 acts as the soft segment and others act as the hard segment. DMPA was used for preparing the SMPU, since DMPA can give higher recovery strain and lower residual strain [11]. At the same time chitin was used as crosslinking agent for enhancing the thermal and mechanical properties of SMPU [16].

2.1. Chemicals 4,4 -Diphenylmethane diisocyanate (MDI), dimethylol propionic acid (DMPA) and 1,4-butanediol (BDO) were procured from Sigma–Aldrich Chemical Co. USA. Polycaprolactone polyol, CAPA 240 (molecular weight 4000 from Solvay Chemicals, Cashur England) and BDO were dried at 70 ◦ C under vacuum for 24 h before use to ensure the removal of all air bubbles and water vapors that may otherwise interfere with the isocyanate reactions. Molecular weight of CAPA 240 was confirmed by applying the procedure reported in ASTM D-4274C. MDI and all other materials were used as received. Dimethylformamide (DMF) and triethylamine were used after being dehydrated with a 4 Å molecular sieve for 2 days. Chitin (Mv = 6.067 × 105 g mol−1 ) was kindly supplied by Iran Polymer and Petrochemical Institute, Iran. Chitin was purified according to already established methods in literature [26]. Its molecular weight was deduced from the intrinsic viscosity, as described in the literature [27]. All the reagents used in this work were of analytical grade. 2.2. Synthesis of shape memory polyurethane (SMPU6) Into a four-necked reaction kettle equipped with mechanical stirrer, heating oil bath, reflux condenser, dropping funnel and N2 inlet and outlet was placed PCL 4000 (3 mmol) and the temperature of the oil bath was increased to 60 ◦ C. Then MDI (5 mmol) and 80 ml DMF were charged to the dried flask and reacted for about 2 h at 80–85 ◦ C; then, DMPA (2 mmol) and MDI (2 mmol) were successively added, and lasted the reaction at the same temperature. Two hours later, BDO (5 mmol) and MDI (6 mmol) were added slowly in another period of 2 h. Then chitin (3 mmol) was added for another 1 h. At last, the neutralization reaction was carried out at 50 ◦ C for 0.5 h, with the addition of TEA (2.2 mmol). The solid content of the product is about 15–20%. The formulation of the prepared SMPU samples is listed in Table 1 and the general structure is presented in Scheme 1. The liquid polymer was casted into a Teflon plate to form a uniform sheet. The synthesized polymer was then placed in a hot air circulating oven at 100 ◦ C and cured for 24 h. The cured sample sheets were then stored for 1 week at ambient temperature (25 ◦ C) and 40% relative humidity before testing. 2.3. Measurements Infrared spectroscopy of the products was performed on a Bruker-IFS 48 Fourier Transform Infrared (FT-IR) Spectrometer (Ettlingen, Germany). The static contact angle () was measured by a sessile drop method at constant room temperature (20 ◦ C) using the

3. Results and discussion

3.1. Molecular characterization FTIR spectra of original chitin and polyurethane extended with 100% BDO (SMPU1) and having different contents of chitin in their formulation (SMPU4, SMPU5 and SMPU6) have been presented in our previous study [16]. The detailed peaks assignments have also been discussed elsewhere [16]. 3.2. Surface morphological studies 3.2.1. Contact angle measurements The surface hydrophilicity/hydrophobicity of biomaterials is very important for biomedical application. The surface properties of the synthesized samples, as characterized by static water contact angle, are reported in Table 2. The contact angle was measured using two different natures of test liquids one polar and the other one non-polar i.e., water and diiodomethane, respectively. The sample with higher contents of DMPA showed the least value of contact angle (SMPU2, SMPU3). This means hydrophilicity of the final PU film increases by incorporating the DMPA into the PU formulations. These variations in contact angles can be ascribed to modification of surface chemistry of the PU samples. Polyol and carboxyl acid are essential in polyurethane manufacturing system. Dimethylol propionic acid (DMPA; CH3 C(CH2 OH)2 COOH), a crystalline solid contains with two kinds of functional groups in one molecule. There are two primary hydroxyl groups and one tertiary carboxyl group which provide function of making hydrophilic polyurethanes. The main advantage of DMPA is that the carboxylic acid is sterically hindered and so preferentially reacts into the backbone through the hydroxyl groups. The values of contact angle using both the test liquid increases as the concentration of chitin in the PU backbone vary from 1.0 mole to 3.0 moles (SMPU4, SMPU5 and SMPU6). As it can be seen, there is a remarkable difference between precursor PU (SMPU4) and PU extended with 3.0 moles of chitin (SMPU6). It is clear from the results that hydrophilic character of the final PU film decreases by increasing the contents of chitin into the final PU formulations. This phenomenon is due to the fact that chitin itself is crystalline polysaccharide and its affinity with water is negligible and is thus very hydrophobic in nature. Therefore by increasing the molar ratio of chitin in the PU backbone, the hydrophobicity of the final PU increases. Chitin should be fairly hydrophilic in nature as it

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Table 1 Sample code designation and formulation of shape memory polyurethane. S. code e

SMPU1 SMPU2 SMPU3 SMPU4 SMPU5 SMPU6 a b c d e

MDI (mol)a

PCL 4000 (mol)b

BDO (mol)c

DMPA (mol)d

Chitin (mol)

10 10 10 11 12 13

6 4 3 3 3 3

4 2 5 5 5 5

– 4 2 2 2 2

– – – 1 2 3

MDI, 4,4 -diphenylmethane diisocyanate. PCL 4000, polycaprolactone polyol, CAPA 240. BDO, 1,4-butanediol. DMPA, dimethylol propionic acid. SMPU1, shape memory polyurethane.

Scheme 1. General structure of the chitin-based shape memory polyurethanes.

contains many polar groups (OH, CH2 OH and NHCOCH3 ) in the structure. On the other hand, it has also been previously reported that the involvement of chitin increases the crystallinity of the synthesized polyurethane [18]. It is well known that increase in crystallinity ultimately increases the hydrophobic character of the polymers. 3.2.2. Water absorption (%) As these polymers are designed to perform and degrade in biological environments, the degradation rate of these polymers

depends on the bulk water absorption ability. Water absorption as a function of time and type of samples are summarized and presented in Table 2. There was no considerable difference in the amount of absorbed water as a function of time. The results revealed that DMPA favours the formation of more hydrophilic polymers. This phenomenon may be due to the fact that the level of dimethylol propionic acid (DMPA) is critical as this controls the resulting particle size. As the amount of DMPA is increased the particle size decreases until eventually a water loving polymer is formed. Also, the increase in the concentration of hydrophilic centres results in

Table 2 Hydrophilicity and swelling data of samples. S. no.

Sample code

Contact angle Watera

1 2 3 4 5 6

SMPU1 SMPU2 SMPU3 SMPU4 SMPU5 SMPU6 a

66.5 51.7 56.9 78.5 86.8 95.2

± ± ± ± ± ±

Water absorption % Diiodomethanea

0.4 0.5 0.2 0.5 0.5 0.3

39.6 42.9 45.1 48.7 51.3 55.4

± ± ± ± ± ±

0.4 0.3 0.2 0.5 0.3 0.4

Each value is expressed as mean ± standard error (S.E.) (n = 7).

Equilibrium degree of swelling

1st day

2nd day

3rd day

5th day

2.77 2.97 2.89 1.19 0.87 0.66

2.79 3.06 2.91 1.22 0.88 0.67

2.80 3.12 2.93 1.23 0.89 0.67

2.81 3.15 2.94 1.25 0.89 0.68

14.88 17.95 15.23 9.15 7.49 6.58

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Table 3 Total surface energy calculated data with Owens–Wendt method and Wu method. Sample code

SMPU1 SMPU2 SMPU3 SMPU4 SMPU5 SMPU6 a

Owens–Wendt method

Wu method

Disperse portion (mN/m)

Polar portion (mN/m)

Total surface energya (mN/m)

34.16 32.45 33.09 36.16 39.05 39.83

9.17 16.15 12.53 4.79 1.84 0.77

41.33 48.60 45.62 40.95 40.87 40.60

± ± ± ± ± ±

0.07 0.08 0.09 0.06 0.08 0.09

Disperse portion (mN/m)

Polar portion (mN/m)

Total surface energya (mN/m)

38.24 36.14 37.78 41.67 43.99 44.07

10.22 16.97 12.59 4.28 1.11 0.47

48.46 51.13 50.37 45.95 45.10 44.54

± ± ± ± ± ±

0.07 0.06 0.08 0.10 0.09 0.06

Each value is expressed as mean ± standard error (S.E.) (n = 7).

a corresponding increase in water sensitivity of the dried polymer film. The results presented in Table 2 clearly show that water absorption of samples decreased with increasing chitin content of samples. The wettability increase is faster in the PU samples extended with zero (SMPU1) or lower (SMPU4, SMPU5) contents of chitin than PU samples extended with higher one (SMPU6). From the evaluation of the results of water absorption and contact angle measurement, we are able to state that DMPA enhance the hydrophilic character while the chitin favours the formation of hydrophobic character of the polymers due to modification of surface chemistry of PU samples. 3.2.3. Equilibrium degree of swelling Physical property such as equilibrium degree of swelling was determined and results are presented in Table 2. The interactions of the final PU films with solvents on the surface were clearly related to the contents of DMPA and chitin into the final polyurethane formulation. The samples with higher contents of DMPA showed less resistance to solvents. These results showed the same pattern and the results were compatible with the results shown by contact angle measurements and water absorption ability. As already presented in the pervious paragraphs, there are two primary hydroxyl groups and one tertiary carboxyl group in DMPA which provide functionality of making ionic centers. The ionic centre in dimethylol propionic acid (DMPA) is responsible of forming hydrophilic polyurethanes and ultimately increases the surface free energy of polymer. The swelling ability of final PU steadily decreases as the chitin ratio increases. This effect can be elucidated by the degree of physical crosslinking and hydrogen bonding in polyurethane structure. This behavior has been confirmed by FTIR and NMR studies of these polymeric materials [16,17]. It is known that chitin is a crystalline polymer of N-acetyl-d-glucosamine monomers; the linked glucosamine rings on chitin can play a role in establishing and increasing H-bonding between soft segment and NH group in the hard segment. Therefore the samples extended with higher contents of chitin have shown much resistant against solvent as compared to the samples having lower contents. 3.2.4. Surface free energies Surface free energy ( s ) was obtained from the contact angle measurements. Owens and Wendt [28] and Wu methods [29] were used to calculate the surface free energies and the data are tabulated (Table 3). It is clear from the results that surface free energies calculated applying the both methods of the sample SMPU2 (the sample having higher contents of DMPA in PU backbone) is much higher than all the other samples and this value decreases with lower (SMPU3) or zero contents (SMPU1) of DMPA. The calculations p of dispersive (sd ) and polar (s ) components of surface free energy have given more detailed information on the surface properties of the samples studied (Table 3). The sample with higher contents of DMPA (SMPU2) has maximum value of polar components while the samples with lower (SMPU3) or zero (SMPU1) contents have least

amount. It has been reported in the established literature that in an aqueous media, the surface tension of the polyurethane with different ionic or mixed ionic groups was increased with increase in concentration of ionic groups, because of the orientation of hydrophobic groups at the air–water interface. The ionic centre in DMPA provides function of making polyurethanes with more polar p (s ) components. The results revealed that surface free energies of the PU much decreased with increase in chitin contents into the PU backbone. Such results may suggest some surface resistance of the PU samples (SMPU6) extended with higher contents of chitin in comparison to the PU samples extended with zero (SMPU1) or lower chitin contents (SMPU4, SMPU5). It is clear from the results that the increase in chitin contents in all the studied samples showed increase in the dispersive components and decrease in the polar components of surface energy simultaneously. This phenomenon is due to the fact that increase in contact angle values leads to decrease in surface energy. The contact angle of a liquid with a solid surface is related to the solid surface energy ( S ), liquid surface tension ( L ) and solid–liquid interfacial tension ( SL ) by the well known Young’s Eq. [31]: cos  =

S − SL L

(1)

This shows that greater the solid surface energy ( S ), or the lower the liquid surface tension ( L ), the lower the contact angle becomes. In other words, one can make a solid surface more wettable either by lowering the surface tension of the liquid or by increasing the surface energy of the solid [32]. 3.2.5. Work of liquid adhesion (WA ) The work of liquid adhesion (WA ) to polymer surface can be calculated from the data of contact angle measurements [33]. The p results of dispersive (WAd ), polar (WA ) and total adhesion (WA ) are presented in Table 4. As can be seen, the work of the water adhesion to PU surface decreases with the decrease of DMPA contents. The sample with higher contents of DMPA showed maximum value as compared to the lower or zeros one. The sample with higher contents of DMPA (SMPU2) has maximum value of polar adhep sion (WA ) while the sample with lower (SMPU3) or zero (SMPU1) contents has least amount. Similarly the dispersive adhesion (WAd ) Table 4 The work of water adhesion to PU films. Sample code

Dispersive adhesion (WAd )

Polar adhesion p (WA )

Total work of adhesiona (WA )

SMPU1 SMPU2 SMPU3 SMPU4 SMPU5 SMPU6

54.9 56.3 55.5 54.0 53.4 52.9

47.6 49.6 48.2 42.8 39.8 37.5

102.5 105.9 103.7 96.8 93.2 90.4

a

Each value is expressed as mean ± standard error (S.E.) (n = 7).

± ± ± ± ± ±

1.0 1.1 1.2 1.1 1.3 1.3

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decreases as the contents of DMPA decrease and chitin increase. The results presented in Table 4 clearly state that the work of the water adhesion (WA ) to PU surface decreases with the increasing chitin contents. These changes are caused mainly by the decrease in polar components, contrary to dispersive one, which somewhat increases by decreasing DMPA or by increasing chitin proportion into the final polyurethane. These changes in the polar and dispersive components attributed to the formation of different dispersive groups on PU film resulting by the incorporation of chitin, increases surface hydrophobicity and decrease the share of polar interactions between polymer and water.

and chitin helps in synthesizing the tailor made polymer according to end use and requirements. It has also been concluded that resulting product has a potential for biomedical implants especially suture.

3.2.6. Surface characteristics and shape memory property The shape memory properties and mechanical properties of these SMPUs have been extensively studied and have been discussed elsewhere [16]. Although SMPU1 and SMPU2 has shown excellent mechanical properties at room temperature, but only the SMPU6 has displayed suitable mechanical properties both at room temperature and above Tm, so it qualifies the criteria of shape memory polymers [16]. The results of the present manuscript revealed that DMPA favors the formation of more hydrophilic polymers contrary to chitin. The incorporation of DMPA increases the polar components while the incorporation of chitin increases the dispersive one. The minimum degree of swelling of SMPU6 among all the samples studied in this investigation also support the impact of higher chitin contents on the improved shape memory properties through extensive soft segment-hard segment miscibility. This improved morphological homogeneity has also been reflected from the improved surface properties of the polymers containing higher percentage of chitin. On the other hand, while keeping the DMPA contents constant and increasing the amount of chitin, the sample SMPU6 has fairly shown the hydrophobic characters. So, it can be stated that the combination of both these helps in synthesizing the tailor made polymer regarding end use and requirements.

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Acknowledgements Financial support of Centre of Excellence for Biopolymers, Iran Polymer and Petrochemical Institute (IPPI), Tehran, Iran and Higher Education Commission (HEC Pakistan) is highly appreciated for the conduct of this work.

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4. Conclusion Chitin-based shape memory polyurethanes (SMPUs) were prepared from polycaprolactone diol 4000 (PCL 4000), 1,4-butanediol (BDO), chitin, dimethylol propionic acid (DMPA), triethylamine (TEA) and 4,4 -diphenylmethane diisocyanate (MDI). The effect of chitin and DMPA contents in polyurethane formulation on surface properties was studied. DMPA provides function of making hydrophilic polyurethanes. Crystalline structure of chitin enhanced the hydrophobicity of the synthesized material. The interactions of polyurethane films with water and diidomethane on the surface were clearly related to the mass ratio of chitin and DMPA contents. Water absorption, and surface free energy, the work of water adhesion and swelling behavior of the synthesized polyurethane samples were affected by varying the DMPA and chitin contents in the chemical composition of the final PU. Finally it can be concluded that the combination of both these two polymers i.e., polyurethane

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