The role of foaming process on shape memory behavior of polylactic acid-thermoplastic polyurethane-nano cellulose bio-nanocomposites

Author’s Accepted Manuscript The Role Of Foaming Process On Shape Memory Behavior Of Polylactic Acid-Thermoplastic Polyurethane-Nano Cellulose Bio-nanocomposites Mohsen Barmouz, Amir Hossein Behravesh www.elsevier.com/locate/jmbbm

PII: DOI: Reference:

S1751-6161(18)30656-8 https://doi.org/10.1016/j.jmbbm.2018.12.021 JMBBM3106

To appear in: Journal of the Mechanical Behavior of Biomedical Materials Received date: 26 April 2018 Revised date: 27 November 2018 Accepted date: 18 December 2018 Cite this article as: Mohsen Barmouz and Amir Hossein Behravesh, The Role Of Foaming Process On Shape Memory Behavior Of Polylactic Acid-Thermoplastic Polyurethane-Nano Cellulose Bio-nanocomposites, Journal of the Mechanical Behavior of Biomedical Materials, https://doi.org/10.1016/j.jmbbm.2018.12.021 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

The Role Of Foaming Process On Shape Memory Behavior Of Polylactic Acid-Thermoplastic Polyurethane-Nano Cellulose Bio-nanocomposites Mohsen Barmouza, Amir Hossein Behravesha* a

Faculty of Mechanical Engineering, Tarbiat Modares University, Tehran, Iran, P.O.Box: 14115-143

Abstract In this study the effects of foaming process on the shape memory properties of Polylactic acid/thermoplastic polyurethane/cellulous-nanofiber bio-nanocomposites were investigated. The samples of cylindrical shapes as well as sheets were manufactured and foamed. The results indicated that while the foaming process presented a microcellular structure, it can cause a tangible increase (up to 40%) in force recovery ratio and an intense reduction (up to 10 times) in actuation force. It is statistically shown that the existence of cellulose nano-fibers within the foamed matrix causes a significant increase in actuation force and reduction in the force recovery ratio. Analytical evaluation on the sheet form samples, in the foamed state using rheological model, was carried out that indicated satisfying description of their shape memory behaviors. It was also demonstrated that there exists significant deviation between the shape memory properties extracted from experimental and analytical assessments. Keywords: Shape memory composite foams; Mechanical properties; Analytical modeling; Creep; Compression moulding *The author to which the correspondence should be addressed: [email protected]

1. Introduction Shape memory polymers (SMPs) are kind of smart materials which they response to external stimulus such as heat, electricity, light, magnetic field, water and solvent (Hwang et al., 2012; Ortega et al., 2007; Rodriguez et al., 2012). Shape memory polymers (SMPs) with capability of changing their temporary shape to a permanent shape over the utilization of an external thermal incentive are known as thermo-responsive SMPs (Huang et al., 2012). SMPs are an important class of materials in medicine, especially for minimally invasive deployment of devices (Behl 1

and Lendlein, 2007). Other interesting applications of the SMPs as medical devices could be adaptive fracture fixator (Zhao et al., 2017) and shape memory stents (Wei et al., 2016; Yakacki et al., 2007; Zarek et al., 2017). Partially crystalline hard segment and a soft amorphous segment of the molecular structure in polymers is the main origin of the shape memory effect in an SMPs (Tobushi et al., 1997). Brownian motion in the soft segment softens the polymeric matrix at the rubbery state; while, at the temperatures below the Tg, the frozen glassy noncrystalline state leads to limit the Brownian motion which suppresses the polymeric matrix deformation (Tobushi et al., 1997). Impeding the Brownian motion during cooling process stiffens the highest portion of the creep strain appeared at the rubbery state as the temperature is increased. Recently, medical applications of biodegradable polymers such as poly (lactide acid) (PLA), poly(ε-caprolactone) (PCL) and their copolymers or blends has been intensively increased (Cha et al., 2014; Gómez-Mascaraque et al., 2014; Henderson et al., 2014; Kalajahi et al., 2016; Lendlein et al., 2005; Paderni et al., 2012; Sammel et al., 2013). PLA is a thermoplastic aliphatic polyester that exhibits excellent biodegradation and biocompatibility which these characteristics make it most suitable materials for medical applications. Recently, PLA are extensively used in biomedical and tissue engineering studies (Chen et al., 2007; Mikos and Temenoff, 2000; Xiao et al., 2012; Zeng et al., 2016). It is also confirmed by various researchers that it could be attractive and more profitable to use mechanical blending in order to add other polymers such as polyamide elastomer (PAE) (Zhang et al., 2009), polycaprolactane (PCL)(Shen et al., 2011), Polyethylene glycol (PEG) (Sungsanit et al., 2012), styrene butadiene styrene copolymer (SBS) (Zhang et al., 2013) to PLA. Recent investigations in the related field of study indicated that thermoplastic polyurethane (TPU) could be suitable candidate for blending with PLA. Due to good mechanical properties, high electricity, durability and biological stability, TPU is a suitable material for medical equipment (Dogan et al., 2017; Simmons et al., 2006). Besides, the TPU/PLA compound biocompatibility has been verified by U.S. Food and Drug administration (FDA) (Oliaei et al., 2015). In the recent decade, shape memory properties of the PLA composition with other polymers has been approved by several researchers such as Lai et al. (Lai and Lan, 2013), Jing et al. (Jing et al., 2015), Song et al. (Song et al., 2015a), Xie et al. (Xie et al., 2015), Y. Zhou et al. (Zhou et al., 2015) and Siahsarani et al. (Siahsarani et al., 2017). Shape memory polymer foams exhibit wide range of potential applications due to a unique combination of low bulk density and high 2

compressibility (Lee et al., 2005). Other than a lower bulk density, creation of cellular structure in a polymeric matrix imparts noticeable advantages such as higher specific surface and higher crystalline structure (Chen et al., 2013; Ji et al., 2013). In a biodegradable foamed sample, a higher specific surface gives rise to a faster rate of degradation that can be considered as a benefit (Lu et al., 2000). Tey et al. (Tey et al., 2001) assessed the effect of long term storage in polyurethane foams on their shape memory properties. They claimed that this product has the ability of full recovery after two months, a characteristics suitable for aerospace equipment. Recently some useful achievements also have been reported on the application of the shape memory polymer foams in biomedical applications (Ortega et al., 2007; Rodriguez et al., 2012; Song et al., 2015a; Song et al., 2015b; Vialle et al., 2009). There exist various methods to get vigorous and constant motivation of the SMPs. One of the most applicable methods is benefiting from the particular functions of the fibrous or particulate as reinforcements. Some of the prominent candidates as fillers will be Carbon and metallic particles, carbon nanotubes (CNTs), carbon nanofibers and glass fibers (Chan et al., 2016; Du et al., 2015; Fonseca et al., 2013; Lu et al., 2013; Ohki et al., 2004; Tan et al., 2015). Cellulose nano-fiber (CNF) is an attractive biopolymer subjected to a lot of works dealing with natural fibers and a sustainable raw material. Moreover, CNF can be a well-established inorganic based reinforcing fiber suitable for commercial composite materials (Kowalczyk et al., 2011; Ma et al., 2016; Visakh et al., 2012; Wang and Li, 2015). Assessment of the analytical prediction methods for the shape memory behavior of the SMPs has been previously studied (Gilormini and Diani, 2012; Liu et al., 2006; Tobushi et al., 2001). 4element rheological model for five different contexts of SMPs including (i) constant stress, (ii) constant strain,(iii) constant stress rate, (iv) constant strain rate, and (v) periodic strain based on the isothermal mechanical response was derived by Bhattacharyya et al. (Bhattacharyya and Tobushi, 2000) and Tobushi et al. (Bhattacharyya and Tobushi, 2000; Tobushi et al., 1997). This research work was dedicated to assess the effect of foaming process on the shape memory behaviors of cellulose nano-fiber reinforced PLA-TPU blends. This is in continue of the research work conducted by the authors on the un-foamed blends of the given compositions, where the 3

samples were manufactured in both cylindrical shell shapes and sheet form (Barmouz and Behravesh, 2017a). As mentioned earlier, the main incentive to this investigation was the ability of foam to present higher surface area that is favorable for degradation. The effect of foaming process on the shape memory behaviors of the PLA-TPU blends, with and without CNFs, was also analytically derived based on the isothermal thermomechanical behaviors of the thin sheet form SMPs. The obtained results bring forward the unique experimental and analytical achievements in the related field of study. 2. Materials and Methods 2.1 Materials Poly(lactic acid)(PLA, 26100-51-6), supplied by Tianjin Glory Tang Technology Co., China with a molecular weight of up to 98000 g/mol, and thermoplastic polyurethane (TPU, shore 85A, 359X) with a molecular weight of up to 220000 g/mol, supplied product of Nanjing HLT Import & Export Co, China, were used as the polymeric components. The bulk densities of the neat PLA and TPU were measured to be about 1.27 and 1.18 g/cm3, respectively. Cellulose nano fiber (CNF), was purchased from Nano Novin Polymer Co., Iran, with fiber diameter of around 100 nm. 2.2 Experimentation 2.2.1 Sample preparation Premixes of PLA, TPU, and cellulose nanofibers (2.5 wt % concentration in water) were prepared utilizing a blender equipped with a specific short blade with a running time of 5 minutes. CNFs were provided in suspension status in water so as to avoid fiber aggregation in the fabricated compounds. Via minimizing the time between the premix preparation and melt blending steps, water absorption into the blends was suppressed. Besides, one could be assured that the water content was evaporated once in contact with the hot chamber of the melt blending unit. The CNF content in the composites was selected in the range of 0 to 6 percent of the dry weight. The PLA and TPU contents were also varied in the ranges of 50-90% and 10-50%, respectively. Before mixing, 12 hours pre-drying process at a temperature of 90°C in a vacuum oven was chosen for the polymeric granules. The blends were prepared, at different weight ratios 4

of PLA, TPU and CNF, utilizing a Brabender internal mixer (Model-W50EHT, 55 cm3 capacity, Germany) equipped with Banbury blades at a rotational speed of 80 rpm and a temperature of 185 °C for 13 min. Similar blending parameters were applied for the samples without CNF in order to compare obtained results. Cylindrical shell shaped SMP samples with the outer diameter and length of 15, and 50 mm, respectively; were manufactured by compression molding process. The cylindrical shell samples were manufactured in tapered shape with variable thickness of 0.3 to 0.7 mm in order to facilitate their extraction from molds. Besides, hot pressing method was utilized to prepare the creep strain and creep strain recovery tests sheet form samples at 185 °C for about 7 min. Then the hot pressed samples was cut to yield thin sheets (0.3 mm in thickness) with length and width of 25 and 6 mm, respectively (namely the cross section of 6×0.3 mm). The samples name was chosen based on the PLA and nano filler contents in such a way that, for instance, the sample which contains 90 wt.% of PLA is coded as P90 and if it contains 2 wt.% of CNFs it is then coded as P90-N2; letter N signifies cellulose nano fibers. High pressure chamber in which the samples were pressurized by CO2 gas was supplied for foaming process. The gas saturation process was completed after a specific time (of gas dissolution into the polymer), and the samples were then removed from the chamber. Supersaturation state was achieved after decompression process, and therefore a thermodynamic instability is occurred. Subsequently, nuclei growth starts once the heating process is started via immersing the samples into the heated glycerol at temperatures above the glass transition temperature. Afterwards, the cell growth inside the sample causes foam expansion in which the foaming temperature and time govern the rate and amount of cells. The foaming parameters comprising saturation pressure and time, foaming temperature and heating time were chosen to be 3.25 MPa, 70 minutes, 100°C and 4 seconds, respectively; for the cylindrical shell samples. For the sheet form samples, those were used for theoretical analysis, only the saturation time was different (25 min.). The reason of selecting different saturation times for the samples was due to the different thicknesses which is the main factor of the gas dissolution (proportional to the square of the thickness (Singh et al., 2004). The foaming conditions can be found given in the previous works by the authors (Barmouz and Behravesh, 2017b). Examples of the foamed and unfoamed SMP samples were shown in Figure 1.

5

Figure 1. (a) foamed and (b) un-foamed cylindrical shell SMPs 2.2.2. Instruments Dynamic mechanical thermal analysis (DMTA) were performed in a single cantilever mode utilizing a dynamic mechanical analyzer DMA 242C, Netzsch, Germany. The samples (25×6×0.3mm) were cut and the properties were measured at a temperature range of 25C to 90C, a heating rate of 5C/min, with a frequency of 1 Hz. Creep strain recovery tests, for analytical study, were carried out by DMA with 30 minutes loading and strain recovery time at isothermal condition. The samples were examined for creep strain recovery test upon a temperature range of 40 to 70°C at 10°C intervals. For programing and recovery processes of the foamed cylindrical shell SMPs, a universal tensile test machine (Instron 4469) was utilized. A PLC control systems was utilized in order to control the temperatures during the foaming and SMPs programming processes. SEM observations were performed after gold sputtering using a Philips XL30 scanning electron microscope with an accelerating voltage of 20 kV. The dimethyl sulfoxide was used as a solvent with intention to detect the CNFs inside the polymeric matrix. 2.2.3 Design of Experiments A response surface methodology (RSM) was used to study the factors affecting force recovery ratio and actuation force of the SMPs. In the present study, it is planned to study the influence of four factors including weight fraction of TPU (TPU%), weight fraction of CNF (CNF%), programming temperature (Pr-Tem), and recovery temperature (Re-Tem) on force recovery ratio and actuation force of the foamed cylindrical shell SMPs. Central composite design with four abovementioned factors and at five levels for each factor was utilized to perform experiments. The Pr-Tem and Re-Tem values and TPU and CNF weight percentages, were chosen to be in the 6

ranges of 10%-50%, 0-6%, 40-70°C, and 40-70°C, respectively. Up to 20°C below and above the PLA’s glass transition temperatures, measured in the different compositions, respectively; was selected as the lower and upper values of the programing temperatures. So, the various samples glass transition temperatures were measured to be in the range of 58-64°C. The SMPs programming and recovery experiment parameters are stated in Table 1. 2.2.4. Shape Memory Characterization The terms Tg, Pr-Tem, and Tl (ambient temperature; up to 30 °C), describing the expressions comprising the glass transition temperature, primary deformation temperature at which the temporarily shape is induced in the polymeric sample (SMP) due to the applied load (or strain), and storage temperature at which the temporary shape of the SMP is fixed, respectively. Foamed cylindrical shell SMP was deformed primarily at Pr-Tem. Since the Tg of the TPU was measured to be far below 0°C, Pr-Tem could be above or well below the Tg of PLA for the different compositions. In the next step, the SMP was cooled to Tl to store temporary shape. Shape recovery effects which describe the SMPs’ force recovery ratio and actuation force were determined by constrained recovery test. Calculation of the force recovery ratios were performed via formula

. In the present study, at first the heating of the SMP samples to Pr-Tem was

performed in their original length and maintained for 30 min prior to loading. Subsequently, the SMPs were subjected to the stretch loading at the elevated temperature to a strain of 10% with a strain rate of 5.5 × 10-4/s. During the cooling process of the SMP until the ambient temperature (Tl), the induced deformation was retained and thereafter the unloading process was proceeded. Finally, in order to measure the actuation force values, at a constrained strain condition the programmed SMPs were heated up with a slight preload. The force recovery measurement was repeated three times for each sample and then the average value was reported. Figure 2a and b show the gripper and furnace which was used to mount and heat up the cylindrical samples, respectively. As it is shown in this figure the gripper has cylindrical part which stays on the hollow part of the tube samples and tightened by clamps. Besides there is tread on the cylindrical part of the gripper to improve the mechanical contact between them. The flat side of the gripper was stays on the tensile test machine gripper to apply the stretch loading on the samples.

7

Figure 2. (a) Gripper for holding and (b) furnace for heating the cylindrical samples 3. Results and discussions 3.1. Characterization of Foamed Cylindrical Shell SMPs Table 1 summarizes the observed dependent variables’ data for the foamed shape memory polymers. Assessment of the actuation force values in this table shows a significant effect of foaming process on the shape memory characteristics. The values of the actuation forces were substantially reduced due to the applied foaming process on the SMPs. It is realized that the maximum value of the actuation force for the foamed SMPs was measured to be 21 N. Also it is found that by applying foaming conditions on the SMPs it is possible to promote force recovery ratio to the value of 92%. Regarding the work reported by Barmouz et al.(Barmouz and Behravesh, 2017a), the value of force recovery ratio was tangibly increased compared to that of unfoamed samples which was measured to be 66%. The main reason for this phenomenon could be attributed to the higher capability of the TPU to perform a recovery reaction over the weakened cellular structure of the PLA. It is worth mentioning that the bulk densities of the foamed samples were measured to be between the 0.25-0.37 g/cm3 in which the lower bulk densities belong to the nano-composites.

8

Table1. Central composite experimental design matrix for the four independent variables used for assessment of shape memory polymer effects Run Order

TPU %

CNF %

Programming Temperature (°C)

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

20 40 20 40 20 40 20 40 20 40 20 40 20 40 20 40 10 50 30 30 30 30 30 30 30 30 30

0 0 4 4 0 0 4 4 0 0 4 4 0 0 4 4 2 2 0 6 2 2 2 2 2 2 2

40 40 40 40 70 70 70 70 40 40 40 40 70 70 70 70 55 55 55 55 25 85 55 55 55 55 55

Recovery Temperature (°C) 40 40 40 40 40 40 40 40 70 70 70 70 70 70 70 70 55 55 55 55 55 55 25 85 55 55 55

Force Recovery Ratio (%)

Actuation force (N)

Standard Deviation (N)

23 30 12 20 45 57 32 41 37 52 43 50 76 92 55 61 10 83 72 26 27 52 14 43 63 62 67

15.5 13.4 21 20 11 10 17 18 15.5 13.5 20.6 20 11 10.8 17 18 12.5 12.5 11 20 15 7 10.4 10.3 10 10 10.2

-1 -1 -2 -1 -2 -1 -2 -2 -2 -1 -1 -2 -2 -1 -1 -2 -1 -2 -3 -2 -1 -1 -2 -2 -3 -1 -2

+2 +1 +1 0 +1 +1 +1 +2 +1 +1 +3 +3 +1 +1 +3 +1 +3 +1 +1 +3 +3 +1 +1 +2 +1 +2 +2

Tables 2 and 3 indicate the analysis of variance results for the second-order regression equation of force recovery ratios and actuation force values, respectively. This analysis was conducted for a significance level of a = 0.05 (confidence level of 95%). It implies that the lower the P-value, the more significant the variable. Based on the regression analysis of force recovery ratio, for the foamed cylindrical shell SMPs, it was found that all SMPs parameters including TPU %, CNF%, Pr-Tem, and Re-Tem had strong impact on the whole model: with P-values smaller than 0.05 (see Table 2). Whereas according to actuation forces values of foamed cylindrical shell SMPs, illustrated in Table 3,; it is recognized that only CNF% and Pr-Tem play significant roles to the 9

response. One explanation could be that adding the TPU to the polymer matrix decreases its strength. As a result of this phenomenon the actuation forces tend to decrease while existence of further elastomeric phase (TPU) inside the SMP matrix improves shape recovery. As a consequence of these opposing influences, existence of the further TPU inside the foamed polymeric matrix did not have significant effects on the actuation force. Regarding the results represented in Table 3, using higher recovery temperature also has some opposing effects on actuation force. On the one hand applying higher recovery temperature weakens the strength of the SMP matrix and surly reduces actuation force. On the other hand it releases higher portion of the stored energy in the SMP, causing enhancement of the actuation forces. Table 2. Variance analysis of force recovery ratios for foamed shape memory polymers Source

DF

F-Value

P-Value

Model

4

12.6

0

Linear

4

12.6

0

TPU%

1

12.25

0.002

CNF%

1

7.39

0.013

Pr-Tem

1

14.04

0.001

Re-Tem

1

16.71

0

Lack-of-Fit

22

27.21

0.036

Pure Error

2

Total

26

Table 3. Variance analysis of actuation force for shape memory polymers Source

DF

F-Value

P-Value

Model

2

20.8

0

Linear

2

20.8

0

CNF%

1

30.68

0

Pr-Tem

1

10.92

0.003

Lack-of-Fit

23

613

0.002

Pure Error Total

2 26

Effects of controlling factors on the mean of the force recovery ratios and the actuation force values in the foamed cylindrical shell SMPs are exhibited in Figures 3.a and b, respectively. The results show that increase in TPU%, Pr-Tem and Re-Tem had a positive effect on the force recovery ratios. Besides, it is signified that adding more CNF negatively affects the force 10

recovery ratios. The existing continuous fibers could act as strong obstacles against SMP recovery and this will be further intensified via using more values of the CNFs. Furthermore, due to the weakened matrix, as a result of foaming process, this prevention role of the CNFs become more intense, and consequently, leads to a significant reduction of the force recovery ratio. The assessment of the governing parameters’ effect on the actuation force (Figure 3.b) indicates that increase in the CNF% promotes the actuation force values. Figure 3.b also illustrates that increase in Pr-Tem has a negative effect on the actuation force. The low capability of the SMP matrix in recovery of elastic energy storage, caused by the weakened foamed structure, could justify this phenomenon.

Figure 3. Main effects plot for (a) Force recovery ratio and (b) Actuation force for the foamed cylindrical shell SMPs

Interactional effects of the TPU%, CNF%, Pr-Tem and Re-Tem on the force recovery ratios and actuation force values of the foamed cylindrical shell SMPs are depicted in Figure 4.a and b, respectively. It is shown that the highest value of the force recovery ratio, up to 83%, could be obtained (Figure 4.a) due to the interactional effects of Pr-Tem and Re-Tem. This could be caused by stiffness reduction of the PLA as a result of foaming process in the compound which 11

contributes to promote the force recovery ratio accomplished by the TPU phase. In addition, highest actuation force value, up to 22.5 N, in the foamed cylindrical shell SMPs has been achieved by interactional effects of the Pr-Tem with CNF% (Figure 4.b). The weakened cellular structure of the polymeric matrix containing thin walls could be the main reason for the low actuation forces in recovery.

Figure 4. Contour plots for (a) Force recovery ratio and (b) Actuation force for the foamed cylindrical shell SMPs DMTA results depicted in Figure 5 for the selected foamed samples including, namely P60 and P60-N4, shows a tangible increase, up to 60%, in normalized storage modulus (E′) as a result of 12

adding CNFs. As shown in this figure the noticeable increase is observed at temperatures below the glass transition temperature (Tg). While for the temperatures above the glass transition temperature the storage modulus growth is attenuated that could be a consequence of weakened adhesion between the matrix and CNFs.

Figure 5. Normalized storage modulus versus temperature for the compounds (a) P60 and P60N4 3.2. The rheological model of a shape memory polymer The constitutive equation stating thermomechanical properties of SMPs is useful instrument to design SMP elements. The overall mechanical properties of polymers could be specified by a standard linear viscoelastic (SLY) model (Lockett, 1972). Whereas, variation of the mechanical properties in the glass transition region cannot described well in the SLV model which refers to the fact that creep recovery is different at temperatures above and below Tg. In order to solve this problem, a slip mechanism due to internal friction is considered for the SLV model. Thus a four-element rheological model for an SMP which involves the slip mechanism is proposed by Tobushi and Bhattacharya et al. (Bhattacharyya and Tobushi, 2000; Tobushi et al., 1997). So as to explain the change in mechanical properties caused by the glass transition, coefficients in the model are expressed by a single exponential function of temperature. Figure 6.a shows a fourelement rheological model for an SMP.

13

Figure 6. Schematic illustration of (a) SLV model for SMP, (b) Creep and creep recovery for the SMPs and (c) an SMP programming and recovery process, (Bhattacharyya and Tobushi, 2000; Tobushi et al., 1997)

The stress-strain relationship in this model is represented as follows: ̇

̇

in which

(1) and are stress and strain, respectively. E, µ, and 𝜆 represent elastic modulus,

viscosity and retardation time, respectively. Figure 6.b plots the isothermal creep strain (ɛc) response versus time for the SMPs. It is illustrated that the creep strain is not completely recovered at temperatures below the Tg, while it could be fully recovered at the temperatures above it. Considering the SMPs creep response to a constant stress at an isothermal condition, the physical significance of the friction element is clearly exhibited. It is worth noting that the irrecoverable strain appears only when the creep strain ( ) exceeds a certain threshold strain

14

value

.

In order to calculate the Irrecoverable strain (friction element), the following equation for the isothermal mechanical response of an SMP at times longer than t1 could be used (Bhattacharyya and Tobushi, 2000): (

)

{ ( )( ( ) ( )

( ))

( ) are positive temperature-dependent parameters in which

where C (T) and ( )

and

( ) ̇( )

( ) ( ) ( ) ̇( )

( )

( )

.

Figure 6.c indicates the shape memory behavior, as illustrated by the previous investigation on the SMP model. For the programing and recovery of the SMPs, four steps are indicated in this figure as follows: 1) loading the SMP at an isothermal temperature of Th (corresponding programming temperature), 2) Cooling the SMP at a constant strain down to a temperature below the Tg (Tl), 3) Unloading the SMP at Tl, and 4) heating up the SMP to a temperature above Tl. As mentioned above the mechanical properties of the SMPs highly depend on their processing temperatures. It is found that the relationship of the log E, µ, 𝜆, ɛL, and C versus Tg/T could be signified by a straight line within the glass transition region. The elastic modulus in the glass transition region is approximated by the following equation: (

)

(3)

or {

(

)}

(4)

In Equation 3, the Eg is the value of the E at T=Tg, and αE represents the slope of a straight line. According to the abovementioned illustration, following equations are introduced as the estimation for the SMP parameters:

𝜆

𝜆

{

(

)}

(5)

{

(

)}

(6) 15

{

( {

)} (

(7) )}

(8)

where Cg, 𝜆g, µg and ɛLg signify the corresponding values at T=Tg, and αc, α𝜆, αµ, αɛL represents the slope of each straight line. Consequently, Equation 1 could be rewritten as follows: ̇( )

̇( ) ( )

( )

( )

(

( )

( )

( )

)

(9)

where the superimposed dots in Equation 9 signifies time derivative. The E(T), µ(T), and 𝜆(T) are defined by the following Equations: ( )

( )

( ),

( )

( ) ( ) ( )

, 𝜆( )

( )

(10)

( )

where the E1(T), E2(T), and 𝜂(T) represent the temperature dependence values of the rheological model parameters. It is worth to note that solutions for the isothermal mechanical response of the SMP under various loading conditions were derived and reported in previous works(Barmouz and Behravesh, 2017a; Bhattacharyya and Tobushi, 2000). In the present study, the isothermal mechanical response of the fabricated foamed SMPs for the composition of 60% PLA- 40% TPU with and without CNFs (coded P60 and P60-N4, respectively) were assessed. As mentioned earlier the examined samples were manufactured in the sheet form. The density of the foamed sheet samples were measured to be up to 0.3 g/cm3 with 0.03 g/cm3 deviation. The temperature-dependent coefficients and related exponents in the exponential function of the temperature were extracted from Equations 4-8 and shown in Table 4 and 5, respectively. As explained in experimental section these values are extracted from the isothermal thermomechanical tests, carried out on the foamed samples of P60 and P60-N4. Table 4. Values of the coefficients derived from Equations 4- 8 at Tg for the compositions Samples ID P60 P60-N4

Tg (°K) 326 326

ɛLg (%) 1.95 1.5

Cg 0.19 0.41

16

𝜆g (s) 705 573

µg (GPa.s) 0.73 1.53

Eg (MPa) 4.9 6.43

Table 5. Calculated exponent values in the exponential function of temperature Samples ID P60 P60-N4

αɛ 19.62 19.57

αc 15.13 11.26

α𝜆 12.61 14.15

αµ 33.9 41.36

αE 17.32 17

Creep strain and creep strain recovery at different temperatures ranging from 40°C to 70°C at 3 °C intervals by applying constant stress of 0.2 MPa for P60 and P60-N4 foamed samples were illustrated in Figure 7.a and b, respectively. Higher creep strain recovery caused by increase in isothermal temperature is demonstrated in Figure 7.

Figure 7. Creep strain and creep strain recovery in the shape memory polymers of (a) Foamed P60 and (b) Foamed P60-N4, extracted from the isothermal thermomechanical tests in constant stress of 0.2 MPa Minimum stress and strain values versus temperature which at the values below them the SMPs will not store the shape memory strain are exhibited in figure 8. It is shown that the minimum stress values for foamed SMPs containing CNFs, in which the shape memory behavior appears, is notably higher than that of foamed SMPs without CNFs (see figure 8.a). Regarding the results illustrated in Figure 8.a, the minimum stress values versus temperature do not constantly increase with an increase in temperature. Indeed at the higher temperatures (up to Tg), due to the severe weakening of the foamed matrix the E and E2 values in corresponding equation, explored by Tobushi et al.(Tobushi et al., 1997), become more close and consequently it causes reduction in the minimum stress values. Figure 8.b signifies that the foamed P60-N4 requires a lower minimum strain values to create a shape memory behavior compared to that of foamed P60. 17

Figure 8. Minimum values of the (a) Strain during constant stress relaxation test, and (b) Stress during constant stress creep test, to create shape memory strain

Figure 9 explains the strain-temperature relationship for the foamed SMPs, derived from the isothermal rheological measurements, at the three different strains of 4.5%, 6%, and 10%, and at the programming temperature of 70°C. Programming and recovery trend of the foamed SMPs for the P60 and P60-N4 are illustrated in Figures 9.a and b in which the Steps 1-4 are corresponding to: 1) constant strain rate loading at Th (programming temperature), 2) cooling down to Tl at a constant strain, 3) unloading the SMPs at an isothermal temperature of Tl, and 4) free stress recovery by heating the SMP to Th. Assessment of Figures 9.a and b specifies that the strain recovery versus temperature in foamed P60-N4 sample is reduced compared to that of foamed P60 sample, while there is not tangible difference between their total strain recovery. Indeed an increase in temperature to a value higher than Tg causes a severe reduction in the strength of the foamed matrix and consequently induces a sharp increase in strain recovery for the foamed P60N4. One explanation could be that for P60-N4, at low temperatures, a high recovery energy is stored due to the strong adhesion between the matrix and CNFs (see figure 5), which acts as an obstacle against the SMPs recovery process. As it is known, using hydrophilic polymers with a high polarity is postulated to be suitable in gaining good dispersion of CNF within the matrix(Barmouz and Behravesh, 2017b; Lee et al., 2014). Figure 10 shows the schematic illustration of this phenomenon in which the aligned CNFs inside the weak foamed structure prevents SMP recovery at the temperatures below the Tg. Afterward, rising the temperature to Tg 18

(or about that temperature) weakens the adhesion between the matrix and CNFs, so a dramatic increase in recovery is occurred.

Figure 9. Strain-Temperature relationship, derived from the thermomechanical tests for the foamed SMPs made of (a) P60 and (b) P60-N4 at the programming temperature of 70°C

Figure 10. Schematic illustration of (a) Recovery of the P60-foamed SMP and (b) Preventing role of the aligned CNFs inside the P60-N4 matrix against shape recovery

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Stress-temperature relationship through the programing and recovery of the foamed P60 and P60-N4 samples at the three different strains (4.5%, 6% and 10%) and the programming temperature of 70°C are provided in Figures 11. The Steps 1-4, indicated in Figure11, are similar to those discussed above, except for Step 4 which describes stress recovery of the foamed SMP under a constrained strain. Figures 11.a and b shows that dramatic increase in the loading stress (Figure 11.b) as compared to that of foamed P60 (Figure 11.a) occurred due to the presence of CNFs within the foamed P60 matrix. It is also seen that with applied strains of 4.5%, 6% and 10%, strong enhancement of the loading stresses up to 75%, 80% and 75%, respectively, were accomplished for the foamed SMPs; via adding 4% CNFs into the foamed P60 SMP. It is also discovered that existence of the CNFs in the foamed P60 blend causes noticeable reduction in the stress recovery ratio (Figures 11.a) as compared to Figures 11.b. Even though it is obvious that the P60-N4 still shows higher values of actuation stress (see Figure 11.c and d). Indeed the stress recovery ratios for the foamed SMPs programmed at different strains of 4.5%, 6%, and 10%, respectively; reduced from 60%, 58%, and 64% in foamed P60 SMPs to 49%, 53%, and 55% in foamed P60-N4 SMPs, respectively. While, the actuation stress values programmed at strains of 4.5%, 6%, and 10%, respectively; for the foamed P60-N4 SMPs are 19%, 40%, and 23% higher than those of foamed P60 SMPs, respectively. Besides, the stress values variation through the cooling stage, represented in Figures 11.a and b, under a constrained strain (Step 2), shows significant growth for the foamed P60-N4 compared to that of foamed P60 SMPs. This phenomenon could be due to the existence of CNFs which could be responsible for the stress relaxation postponement during the cooling process (Siengchin and Karger-Kocsis, 2010).

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Figure 11. Stress-Temperature relationship, derived from thermomechanical tests for the foamed SMPs made of (a) P60 and (b) P60-N4 at the programming temperature of 70°C, (c) and (d) Step 4 (actuation stress) for the P60 and P60-N4, respectively Figure 12 indicates the surface morphology of the foamed P60 and P60-N4 SMPs. The surface morphology of the foamed P60 (Figure 12.a) illustrates the formation of the fine cells (around 10 µm) throughout the matrix structure. Figure 12.b signifies the pulling CNFs out from the polymeric matrix which can cause an improvement in stiffness and strength.

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Figure 12. The surface morphology of the foamed SMPs made of (a) P60 and (b) P60-N4

Theoretical and experimental results for both foamed P60 and P60-N4, programmed at the temperature of 70°C and the strain of 10% are compared in Figure 13. There exist some differences between the theoretical and experimental results for the foamed P60 and P60-N4 SMPs. As discussed earlier, the geometry of the samples used for the experimental and theoretical assessments were different: cylindrical shell shape and sheet form, respectively; for experimental and theoretical studies. As a result, it seems to be reasonable that the geometry difference and consequently different patterns of cell formation during expansion process may cause a tangible deviation between the experimental and theoretical results (figure 13.a) for the foamed P60 SMPs. For the foamed P60-N4 SMPs (Figure 13.b), the additional reason could be the non-identical orientation of CNFs inside the cylindrical and sheet form samples which brings about more intense deviation between the experimental and theoretical results.

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Figure 13. Comparing the stress-temperature relationship obtained from the theoretical and experimental results for the foamed SMPs made of blend (a) P60 and (b) P60-N4 at the programming temperature of 70°C

Conclusions: In this study, the effect of foaming process on the shape memory behavior of the PLATPU/Cellulose nanofiber bio-nanocomposites was accomplished both theoretically and experimentally. The cylindrical shell shape samples were successfully made, foamed, and used for the experimental investigation. Statistical analysis was conducted on the experimental investigation using the response surface design method. The samples in sheet form were foamed to study the SMPs, analytically, via analyzing their isothermal thermomechanical properties. Analytical assessment on the shape memory behaviors of the foamed PLA-TPU/CNFs nanocomposites exhibits similar achievements to that in the experimental investigation. The results conclude the followings: 1- Foaming process caused a considerable reduction in the actuation force values of the cylindrical shell SMPs made of the PLA-TPU/CNFs nanocomposites (up to 10 times). 2- Creating a cellular structure in the PLA-TPU cylindrical shell SMPs increased the force recovery ratio (up to 39%) compared to that of the unfoamed SMPs. 3- Presence of the CNFs in the foamed cylindrical shell form PLA-TPU led to an increase in the actuation force values and a reduction in the force recovery ratios. 23

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